A technology that significantly improves ethanol gas sensing performance by combining five different metal elements in a single oxide has been developed.
A research team led by Professor Ho Won Jang of the Department of Materials Science and Engineering at Seoul National University College of Engineering developed an ethanol gas sensor based on a high-entropy oxide combining indium, tin, iron, zinc, and tungsten, and analyzed the surface reaction characteristics responsible for its enhanced performance. When exposed to 50 ppm ethanol at 300°C, the sensor exhibited a response 12.6 times higher than that of the low-entropy oxide used for comparison. The findings were published in the international journal Advanced Science.
Metal oxide-based gas sensors detect gases by measuring changes in electrical resistance that occur when gas molecules react with the sensor surface. Because of their simple structures and low fabrication costs, they are being studied for applications in a wide range of areas, including indoor air-quality management, food-condition monitoring, and industrial-site monitoring.
Conventional sensors using a single type of metal oxide, however, have a limited number of surface sites available for gas reactions, making it difficult to simultaneously improve their ability to detect small amounts of gas and distinguish a specific gas from others. Researchers have explored approaches such as adding other metal elements or coating sensor surfaces with noble metals such as gold, platinum, and palladium, but these methods have limitations including the stability of added elements and the high cost of noble metals.
The research team instead focused on "high-entropy design," which combines multiple metal elements within a single material. High-entropy oxides are materials in which several types of metal elements are mixed in similar proportions within a single oxide structure. When different elements are combined, the atomic environments become more diverse, potentially altering the properties of surface sites where gas molecules adsorb and react.
The researchers synthesized high-entropy oxide nanostructures in which the five metal elements were uniformly distributed and compared their sensing performance under identical conditions with oxide sensors having low or medium entropy.
The results showed that when exposed to 50 ppm ethanol at 300°C, the high-entropy oxide sensor exhibited a response of 5,135%. The low-entropy oxide showed a response of 408%, while the medium-entropy oxide recorded 654%, meaning that the response of the high-entropy oxide was approximately 12.6 times and 7.9 times higher, respectively. Sensor response is a measure of how much the electrical resistance changes when the sensor is exposed to the target gas. These values are also reported in the published paper.
The sensor had a response time of 3 seconds and a recovery time of 73 seconds after the ethanol was removed, demonstrating faster response and recovery than the comparison oxides. The theoretical limit of detection calculated from the response signal at different ethanol concentrations was 4.8 ppb. This represents the minimum detectable concentration estimated from measurement data and is distinct from a result in which sensing performance was directly verified at that concentration.
The researchers traced the improved performance to changes in oxygen adsorbed on the material surface and in its defect structure. The sensor detects changes in electrical resistance that occur when oxygen adsorbed on its surface reacts with ethanol. The high-entropy oxide contained more adsorbed oxygen and more oxygen vacancies than the low-entropy oxide. Oxygen vacancies are sites in an oxide structure where oxygen atoms are missing, and they affect gas adsorption and reactions. The researchers concluded that the combination of different metal elements altered these surface properties and promoted reactions with ethanol. The published study likewise identifies increased chemisorbed oxygen and oxygen vacancies as key factors in the enhanced response.
The researchers also evaluated the sensor's long-term stability and operation under high humidity. The sensor maintained its ethanol sensing performance during a six-month stability evaluation and was capable of repeated sensing even under conditions of 90% relative humidity. However, because its optimal operating temperature is 300°C, further research will be needed for practical applications to lower the operating temperature and improve both its selectivity in distinguishing ethanol from other gases and its tolerance to changes in humidity.
The study is significant in that it analyzes the relationship between elemental composition, the resulting structural and surface properties of the material, and gas sensing performance, while proposing a direction for designing sensor materials that does not rely solely on noble-metal catalysts. The findings are expected to contribute to the development of tailored gas-sensing materials for applications including indoor air-quality management and food and industrial monitoring.
Professor Jang, who supervised the research, said, "This study confirmed that entropy engineering, through controlling combinations of multiple elements, can alter the surface reactions of oxide materials and improve ethanol sensing performance." He added, "Going forward, we plan to explore diverse elemental combinations to develop materials that respond selectively not only to ethanol but also to a variety of gases, and on that basis expand our research toward sensors capable of detecting different gases."
Gi Baek Nam and Jin Ho Seo, integrated M.S./Ph.D. students in the Department of Materials Science and Engineering at Seoul National University, participated in the study as co-first authors. Nam studies the synthesis and properties of inorganic materials, including metal oxides and sulfides, while Seo studies the synthesis of high-entropy materials and their application as catalysts for water splitting. Both researchers plan to continue related research as postdoctoral researchers after earning their Ph.D. degrees.
The research was supported by the Ministry of Science and ICT and the National Research Foundation of Korea (RS-2024-00421181, RS-2024-00405016), the Ministry of Science and ICT's InnoCORE program (1.250021.01), the National Research Council of Science & Technology (GTL25021-230), the Rural Development Administration's Cooperative Research Program (PJ01706703), the Basic Science Research Program of the Ministry of Education and the National Research Foundation of Korea (RS-2025-25396663), and the BK21 FOUR program, among others. The Inter-University Semiconductor Research Center and the Institute of Engineering Research at Seoul National University provided research facilities. These funding sources and facility support are also specified in the published paper.
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