Scientific Papers

Simple synthesis of Cu/Cu2O/CuO composites with dual adsorption capacity for conductometric NO2 detection


NO2 is a kind of reddish-brown, pungent and toxic gas [1], which is mainly produced from the emission of automobile exhaust, leakage of transportation and as a by-product of high-temperature combustion reactions in industrial processes [2], causing great harm to the environment and human life and health. It is reported that short-term exposure to NO2 gas of medium and high concentration (>50 ppm) can immediately cause harm to human life [3]. Even if exposed to low doses (1 ppm), NO2 may cause a series of health problems such as headache and organ irritation [4]. The exposure limit recommended by the National Institute of Occupational Safety and Health is 1 ppm [5]. In addition, NO2 can react with other chemicals in the surrounding atmosphere to generate acid rain, ground ozone and photochemical smog, which are great harm to the ecosystem [6]. In order to effectively reduce the related negative effects caused by NO2, the United States Environmental Protection Agency (EPA) stipulates that the toxicity limit of NO2 to environmental problems is 53 ppb [7]. Therefore, it is very necessary for highly sensitive detection of low concentration NO2.

Recent reports have shown that the gas sensitive reaction of materials to NO2 mainly includes the adsorption process of the material itself to the target gas (NO2) and the interaction process between the chemically adsorbed oxygen on the material surface and the target gas (NO2). However, the current research usually only improves one of the processes by modifying the materials to further enhance the detection ability of the materials for NO2. For example, the nanoflower-like NiO/CuO composite and NiO/In2O3 composite improved responses to NO2 due to the increase of surface adsorbed oxygen of the material [8], [9]. Another example is the Ni doped SnO2 material synthesized by Li et al., which showed a strong adsorption capacity for NO2, thereby improving the response of material to NO2 [10]. However, it is rare to synthesize a material that can simultaneously improve the adsorption capacity of NO2 and the surface adsorbed oxygen content, so designing and synthesizing a material with both characteristics to enhance the detection ability of NO2 will be a meaningful and groundbreaking work.

CuO is a typical p-type semiconductor oxide material with narrow band gap (1.2 eV) [11], which has also shown good sensing performance in NO2 detection [12], [13], [14]. But most of reports improve their sensing performance to NO2 by improving one of the processes. For example, Wang et al. synthesized Cu2O/CuO composite material, in which the modification of Cu2O resulted in a higher surface adsorbed oxygen content than pure CuO, and thus a much higher response to NO2 than CuO [15]. Li et al. synthesized CuO/rGO materials, which exhibited excellent NO2 sensing performance due to their large specific surface area and more interfaces, which are conducive to the adsorption of NO2 [16]. Inspired by the above improved materials, the synthesis of a CuO based NO2 gas sensing material that can simultaneously enhance the adsorption of NO2 gas and the surface adsorbed oxygen content may enhance the detection ability of sensing material to NO2 gas. Some reports indicated that the presence of Cu in the materials, such as SiO2 and activated carbons, can promote the adsorption of NO2 gas on the materials [17], [18], [19]. Therefore, synthesizing CuO-based composite materials containing Cu may improve the sensing performance of NO2 gas. In recent years, the synthesis of metal oxide gas sensing materials using MOFs as self-sacrificial templates has become a current research hotspot, especially by controlling appropriate pyrolysis conditions, metal/metal oxide hybrids can be synthesized [20]. Due to the multivalent nature of Cu, if Cu MOF is used as a precursor, it is possible to obtain dual or even multi component copper oxide composites by only controlling pyrolysis conditions. At the same time, exploring the relationship between components, structure, and performance, revealing the reasons for improving performance, will be of great significance for the future design and synthesis of materials with good gas sensing performance.

Based on the inspiration of the above research, regular nano-sized octahedral HKUST-1 precursor was prepared by solvothermal method. A series of copper oxide-based materials were obtained by controlling the sintering temperature: Cu/Cu2O/CuO, Cu2O/CuO, CuO, and their NO2 sensing performance was investigated in detail. Among them, the porous octahedral Cu/Cu2O/CuO composite with more oxygen vacancies exhibits the best response to 1 ppm NO2 (S=14.4) at 50 °C, and excellent selectivity, long-term stability and moisture resistance. The detection limit is as low as 50 ppb. The Cu/Cu2O/CuO material achieves the design of simultaneously improve the adsorption capacity of NO2 and surface adsorbed oxygen content, and has good NO2 sensing performance. Moreover, the Cu/Cu2O/CuO sensor can also detect dibutylamine at 170 °C. This result enables sensitive detection of different gases by a single sensor through regulating the working temperature.



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