INTRODUCTION
Ethanol is a colorless, transparent liquid at room temperature and standard atmospheric pressure, widely used in industrial production, food manufacturing, healthcare, and fuel industries[1]. It is highly flammable and volatile, and if undetected leaks occur, they can pose significant risks to surrounding personnel, particularly in the presence of open flames, potentially leading to irreparable damage. Therefore, developing a gas sensor with high response sensitivity, a low detection limit, and rapid response capability for ethanol detection is crucial. Gas sensors are currently categorized based on their working principles, including semiconductor, electrochemical, optical, solid electrolyte, and catalytic combustion sensors[2-6]. Among these, semiconductor gas sensors stand out for their compact size, low cost, and high sensitivity, making them ideal for various applications and extensively researched[7]. Metal oxide semiconductors, in particular, are commonly used in gas sensors due to their stability, high detection accuracy, and cost-effectiveness. For instance, oxides such as In2O3[8], Co3O4[9], Cu2O[10], and SnO2[11] have been reported as effective gas-sensing materials. Additionally, zinc oxide (ZnO) shows great promise in gas detection owing to its high electron mobility, diverse morphological control, and excellent thermal stability.
ZnO is a semiconductor with a direct bandgap, characterized by a high exciton binding energy, high electron mobility, and a wide bandgap. Nanostructured ZnO exhibits excellent optical, piezoelectric, and electrochemical properties, making it widely used in the fabrication of various sensors, including photoelectric, pressure, temperature, and gas sensors[12-15]. In recent years, ZnO gas sensors derived from metal-organic frameworks (MOFs) have garnered significant attention due to their large specific surface area and abundant channel gaps. A large surface area provides abundant adsorbable sites for target gases, promoting the adsorption and reaction rates, thereby enhancing the material’s gas-sensing performance. ZIF-8, a common MOF material created through coordinating zinc ions with the organic ligand 2-methylimidazole, is valued for its large surface area, structural stability, significant porosity, and ease of synthesis. ZIF-8 can be easily oxidized to form ZnO through calcination in air[16]. Ren et al. obtained porous ZnO nanocubes derived from MOFs by pyrolyzing ZIF-8 at 500 °C, achieving a response value of 51.41 for 1 ppm NO₂ at 200 °C - a significant improvement compared to similar work[17]. In addition, heterogeneous metal element doping and noble metal modification are also effective methods for enhancing the gas-sensing properties of materials. Bulemo successfully synthesized Ga-doped ZnO strip materials that exhibited a specific surface area of up to 68.5 m2/g using the electrospinning method. At a working temperature of 400 °C, the response value was 21 to 20 ppm acetylene gas, and the detection limit reached 0.2 ppm, which is significantly improved compared to pure ZnO materials[18]. Dai et al. synthesized Au-modified ZnO rod-like nanoflowers, which demonstrated an extremely fast response time (15 s), a high response value (138 for 100 ppm), and a low detection limit (1 ppm) for ethanolamine detection[19]. Despite these advancements, current ZnO-based gas sensors still face challenges, including high operating temperatures and room for improvement in sensitivity. Due to multiple oxidation states of cobalt (Co) (including Co2+and Co3+), cobalt ions release more electrons than Zn2+ ions when doped into ZnO. The ionic radius of Co2+ is similar to that of Zn2+, meaning that the lattice distortion caused by the substitution of Co2+ for Zn2+ is relatively small[20]. Additionally, the work function of the noble metal Au (5.1 eV) is higher than that of ZnO (4.45 eV), which can produce an electron sensitization effect. Au also exhibits a chemical sensitization effect, both of which significantly enhance the gas response of ZnO[21]. Therefore, in this study, the combination of Co doping and Au loading was used to further improve the gas-sensing performance of MOF-derived ZnO nanostructures.
In this study, the MOF-derived Porous Au@Co-ZnO nanostructure was successfully fabricated, retaining the structural characteristics of ZIF-8 with a large specific surface area and high porosity. Additionally, the gas sensing performance was significantly enhanced compared to pure MOF-derived ZnO due to Co doping and gold nanoparticle modification. The Au@Co-ZnO nanostructure was prepared by calcining ZIF-8 in air after Co doping and gold ion exchange. In 100 ppm ethanol gas, the optimal operating temperature for the Au@Co-ZnO sample was 140 °C, which is 40 °C lower than the 180 °C required for the pure ZnO sample. Moreover, at the same operating temperature (140 °C), the Au@Co-ZnO sample exhibited a response value of 205.3, which is 28.9 times higher than the response value of 7.1 from the pure ZnO sample to 100 ppm ethanol. Additionally, the Au@Co-ZnO sample exhibited excellent stability and selectivity for ethanol. These results confirm the effectiveness of combining Co doping with Au nanoparticle modification, providing a promising strategy for enhancing the gas-sensing property of other metal oxide materials and advancing the application of MOF-derived materials in gas detection.
