Thickness Optimization of Highly Porous Flame-Aerosol Deposited WO3 Films for NO2 Sensing at ppb

Flame Spray Pyrolysis Synthesis of WO3 Sensing Materials: Effects of Flame Parameters on Particle Size Distribution and NO2 Sensing Performance

In this study, the flame spray pyrolysis (FSP) technique was employed to produce WO₃ nanoparticles, which were subsequently used as sensing materials for NO₂ sensors. To enhance the sensing performance, the effects of flame parameters on the particle properties and sensing performances for 150-1200 ppb NO₂ at 125 °C were investigated. The results indicate that WO₃ particles with an average crystal size of about 10-20 nm and a standard deviation of about 3-7.5 nm were generated by controlling the precursor and dispersion oxygen flow rate of FSP. Based on the evaluation of NO₂ sensing performance, WO₃ sensing materials synthesized under the 3/5 flame condition exhibited better sensitivity than sensors made under other flame conditions. In summary, the FSP method and the optimization of flame synthesis parameters could be an effective strategy to prepare the sensing materials with high sensing performance.

Temperature-Modulated Selective Detection of Part-per-Trillion NO2 Using Platinum Nanocluster Sensitized 3D Metal Oxide Nanotube Arrays

Semiconductor chemiresistive gas sensors play critical roles in a smart and sustainable city where a safe and healthy environment is the foundation. However, the poor limits of detection and selectivity are the two bottleneck issues limiting their broad applications. Herein, a unique sensor design with a 3D tin oxide (SnO₂ ) nanotube array as the sensing layer and platinum (Pt) nanocluster decoration as the catalytic layer, is demonstrated. The Pt/SnO₂ sensor significantly enhances the sensitivity and selectivity of NO₂ detection by strengthening the adsorption energy and lowering the activation energy toward NO₂ . It not only leads to ultrahigh sensitivity to NO₂ with a record limit of detection of 107 parts per trillion, but also enables selective NO₂ sensing while suppressing the responses to interfering gases. Furthermore, a wireless sensor system integrated with sensors, a microcontroller, and a Bluetooth unit is developed for the practical indoor and on-road NO₂ detection applications. The rational design of the sensors and their successful demonstration pave the way for future real-time gas monitoring in smart home and smart city applications.

Nanoplasmonic NO2 Sensor with a Sub-10 Parts per Billion Limit of Detection in Urban Air

Urban air pollution is a critical health problem in cities all around the world. Therefore, spatially highly resolved real-time monitoring of airborne pollutants, in general, and of nitrogen dioxide, NO₂, in particular, is of utmost importance. However, highly accurate but fixed and bulky measurement stations or satellites are used for this purpose to date. This defines a need for miniaturized NO₂ sensor solutions with detection limits in the low parts per billion range to finally enable indicative air quality monitoring at low cost that facilitates detection of highly local emission peaks and enables the implementation of direct local actions like traffic control, to immediately reduce local emissions. To address this challenge, we present a nanoplasmonic NO₂ sensor based on arrays of Au nanoparticles coated with a thin layer of polycrystalline WO₃, which displays a spectral redshift in the localized surface plasmon resonance in response to NO₂. Sensor performance is characterized under (i) idealized laboratory conditions, (ii) conditions simulating humid urban air, and (iii) an outdoor field test in a miniaturized device benchmarked against a commercial NO₂ sensor approved according to European and American standards. The limit of detection of the plasmonic solution is below 10 ppb in all conditions. The observed plasmonic response is attributed to a combination of charge transfer between the WO₃ layer and the plasmonic Au nanoparticles, WO₃ layer volume expansion, and changes in WO₃ permittivity. The obtained results highlight the viability of nanoplasmonic gas sensors, in general, and their potential for practical application in indicative urban air monitoring, in particular.

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

In this review paper, we provide an overview of state-of-the-art Pd-based materials for optical H2 sensors. The first part of the manuscript introduces the operating principles, providing background information on the thermodynamics and the primary mechanisms of optical detection. Optical H2 sensors using thin films (i.e., films without any nanostructuring) are discussed first, followed by those employing nanostructured materials based on aggregated or isolated nanoparticles (ANPs and INPs, respectively), as well as complex nanostructured (CN) architectures. The different material types are discussed on the basis of the properties they can attribute to the resulting sensors, including their limit of detection, sensitivity, and response time. Limitations induced by cracking and the hysteresis effect, which reduce the repeatability and reliability of the sensors, as well as by CO poisoning that deteriorates their performance in the long run, are also discussed together with an overview of manufacturing approaches (e.g., tailoring the composition and/or applying functionalizing coatings) for addressing these issues.