Magnesium-doped zinc oxide nanopowder gas sensor device for nitrogen dioxide detection

Abstract

A nitrogen dioxide gas sensor device consisting of a sensor layer of magnesium-doped zinc oxide nanopowder produced by a sol-gel self-combustion process and deposited on interdigital electrodes on a Si / SiO2 substrate.

Description

Application area of ​​the invention

[0001] The present invention relates to the field of semiconductor gas sensors and nanomaterial-based sensor systems, in particular the production of Mg-doped ZnO nitrogen dioxide gas sensors based on nanopowders for applications in the field of environmental monitoring. Background of the invention

[0002] The detection and monitoring of toxic gases such as nitrogen dioxide (NO2) are becoming increasingly important due to their harmful effects on human health and the environment. Conventional gas sensors, for example, systems based on metal oxide semiconductors, often exhibit low selectivity, high operating temperatures, and long response and recovery times. Zinc oxide (ZnO) has been intensively investigated as a promising gas sensor material due to its large band gap, high chemical stability, and good electrical conductivity. However, pure ZnO shows limited sensitivity and selectivity at low NO2 concentrations. Doping ZnO with suitable elements allows for the effective modification of its electronic structure and surface properties, thereby improving its gas sensor performance.Doping with magnesium (Mg) has proven to be an effective approach to increase the oxygen vacancy concentration and modify the bandgap energy of ZnO. Despite this potential, there is a need for a reproducible and scalable method for synthesizing Mg-doped ZnO nanopowders with controlled morphology and composition, enabling their integration into sensors and thus achieving high-performance NO₂ detection at relatively low operating temperatures with fast response and recovery characteristics. Summary of the invention

[0003] Nitrogen dioxide (NO2) gas sensors based on magnesium-doped zinc oxide (Znı- x Mg xO2, where 0 ≤ x ≤ 0.10). The process utilizes a sol-gel self-combustion synthesis to produce homogeneous Mg-doped ZnO2. Nanopowders with a fine grain size and uniform distribution are synthesized. The powders are annealed at controlled temperatures to achieve a stable hexagonal wurtzite crystal phase and subsequently deposited onto interdigital electrode structures (IDEs) on Si / SiO2 substrates to form the sensor layer.

[0004] The magnesium doping level was optimized to achieve maximum sensitivity and selectivity for NO₂ gas. Experimental investigations show that a magnesium doping concentration of 5% provides the best sensor performance, achieving an 81% response to 5 ppm NO₂ at 200 °C with short response (27 seconds) and recovery times (32 seconds). The magnesium incorporation promotes the formation of oxygen vacancies, increases the number of surface hydroxyl groups, and widens the band gap from 3.2 eV to 3.4 eV, resulting in improved charge carrier dynamics and superior gas sensor performance. The invention thus offers a cost-effective, reproducible, and scalable manufacturing process for high-performance NO₂ gas sensors suitable for environmental safety and industrial emissions monitoring. Detailed description of the invention

[0005] The invention discloses a device and a manufacturing process for Mg-doped ZnO, a nitrogen dioxide gas sensor based on nanopowder. The synthesis begins with the preparation of a sol-gel solution containing zinc nitrate hexahydrate and magnesium nitrate hexahydrate as metal precursors, citric acid as fuel, and deionized water as solvent. The molar ratio of citric acid to the total metal ions is maintained at 1:1 to ensure complete combustion and gel formation. The resulting sol is continuously stirred at 80 °C until a viscous gel forms, which then undergoes self-combustion to yield a bulky, flaky powder.

[0006] The freshly synthesized, Mg-doped ZnO powders are calcined in air at 500–600 °C for two hours to remove organic residues and promote the formation of a crystalline wurtzite-ZnO phase. X-ray diffraction (XRD) confirms the single-phase hexagonal structure without secondary impurity phases, while field emission scanning electron microscopy (FESEM) reveals uniformly distributed nanoparticles with an average size of 30–50 nm. Energy-dispersive X-ray spectroscopy (EDX) confirms the homogeneous Mg distribution within the ZnO lattice.

[0007] The sensor element is fabricated by depositing annealed nanopowders onto interdigital gold electrodes structured on a silicon wafer with a thermally grown SiO2 insulating layer. Deposition is achieved via drop coating or screen printing, followed by drying and mild annealing to improve adhesion and contact between the sensor layer and the electrode surface. Electrical connections are then made to enable resistance-based gas sensing measurements.

[0008] The sensor characteristics were investigated in a controlled gas chamber at varying NO2 concentrations of 1 to 10 ppm and operating temperatures between 150 °C and 250 °C. The sensor response, defined as the ratio of resistance in air to resistance in the presence of NO2 gas, shows maximum sensitivity at 200 °C and a 5% magnesium doping. The response and recovery times are 27 and 32 seconds, respectively, indicating rapid adsorption-desorption kinetics.

[0009] The improved performance of the Mg-doped ZnO sensor is due to the modification of its surface and electronic properties caused by the Mg substitution. The presence of Mg 2+The incorporation of magnesium ions increases the number of oxygen vacancies, which act as active adsorption sites for NO₂ molecules, thus leading to higher surface reactivity. Furthermore, the incorporation of magnesium slightly increases the band gap energy, thereby improving electron mobility and charge separation during gas interactions. Fourier-transform infrared spectroscopy (FTIR) confirms the presence of surface hydroxyl groups that contribute to the gas adsorption mechanisms.

[0010] Selectivity tests with other gases such as CO and NH3 show negligible reactions compared to NO2, demonstrating high selectivity. The sensor exhibits good reproducibility and stability over multiple operating cycles and shows only negligible drift even in long-term operation. The cost-effective sol-gel self-combustion process enables easy scaling for the production of large-area sensor arrays, making the device suitable for integration into portable gas monitoring systems and IoT-based environmental monitoring applications.

[0011] Overall, the invention provides a robust, cost-effective, and highly sensitive NO2 gas sensor based on Mg-doped ZnO nanopowders with adjustable structural and electronic properties, achieved through controlled doping and temperature conditions.

Claims

[1] A nitrogen dioxide gas sensor device consisting of a sensor layer of magnesium-doped zinc oxide nanopowder produced by a sol-gel self-combustion process and deposited on interdigital electrodes on a Si / SiO2 substrate. [2] Sensor device according to claim 1, wherein the magnesium doping concentration is in the range of 0 to 10 mol%, wherein an optimal doping concentration of 5% gives the maximum sensitivity. [3] Sensor device according to claim 1, wherein the sensor layer has a hexagonal wurtzite crystal structure with increased oxygen vacancies and an enlarged band gap in the range of 3.2 to 3.4 eV. [4] Sensor device according to one of the preceding claims, wherein the device has a sensitivity of at least 80% to 5 ppm NO2 gas at 200 °C, with response and recovery times of less than 35 seconds.