Micro-SENSors based on metal oxide sensitive layers and Metal Organic Frameworks for AIR quality monitoring at low cost - 2020 – SensMOFAir-2020

The SensMOFAir-2020 project is based on a rigorous scientific methodology combining three complementary research areas to create smarter and more efficient gas sensors. The goal is to overcome the current limitations of sensors, particularly their lack of selectivity.

1. Formulating New Sensitive Layers It all starts with the gas-sensitive layer.

The team chose to work on nanostructured ZnO:Ga thin films. This composition was chosen because it is both sensitive to the targeted polluting gases and compatible with the next step, which is partial conversion into MOFs (Metal Organic Frameworks). The CIRIMAT team, experts in sputtering, guarantees precise control of the film microstructure. These nanostructured films have a high surface-to-volume ratio, which is essential for effective detection. The deposition technologies used make these thin films compatible with the microplatforms developed by LAAS.

2. Developing Smart Filtration Layers (MOFs)

The most innovative step involves adding an ultra-selective "barrier": MOF (Metallic Organic Materials) structures. MOFs are crystalline materials with pores calibrated at the molecular level. They act like molecular sieves to prevent the diffusion of certain molecules (such as steam) and to pre-concentrate the desired pollutants, thus improving the sensor's selectivity. MOFs are not simply deposited; they are obtained by surface conversion of the metal oxide (IEM process) directly onto the sensor microplatforms. For example, ZnO can be converted into ZIF-8. The IRCELYON team can expand the range of available MOFs by adjusting their porosity and adsorption characteristics (for example, by inserting metal nanoparticles or grafting chemical functions).

3. Measurement in Temperature Cycling Mode

A new material is not enough; it must be integrated into a smart platform. The sensitive layers (with or without MOFs) are integrated onto heated microplates (developed by LAAS). A simple process using a metal mask has been developed for this integration. The sensors are then tested at a single constant temperature or in temperature cycling mode. The latter records the transient response or the difference in resistance at two different temperatures. This operating method also increases the sensitivity and selectivity of the sensors.

In summary, the method combines innovative material chemistry (ZnO:Ga and MOF) with microsystem engineering (micro-hotplates and cycled temperature), creating a synergy for both sensitive and highly selective gas detection.

Next-Generation Smart Sensors: Greater Selectivity and Record NO2 Detection

The SensMOFAir project has taken crucial steps toward creating next-generation air pollution sensors, specifically targeting NO2 (nitrogen dioxide), a toxic gas even at very low concentrations. The major achievements are in three areas: optimization of the sensitive layer, integration of smart filters (MOFs), and development of a reliable manufacturing process.

1. Record Performance of the NO2 Sensor (Axis 1)

The heart of the sensor is a nanoscale film of gallium-doped zinc oxide (GZO). The raw films are metastable. Annealing at 600°C was identified as optimal, as it tripled the sensor's sensitivity to 100 ppb NO2. This improvement is due to the increase in the relative concentration of oxygen vacancies in the film, which act as preferential sites for NO2 capture. By optimizing the film thickness (less than 25 nm) and substrate roughness, the sensor achieves a response of 17 in less than 10 minutes for 100 ppb NO2.

2. MOF Smart Filters (Axis 2)

To improve selectivity, Metal-Organic Frameworks (MOFs) are developed to act as filters. The work optimized the conversion process from GZO to MOF ZIF-8. An optimal 3:1 methanol/water ratio is required to form continuous and adherent ZIF-8 membranes on the GZO sensor. However, annealing at 600°C, which improves the GZO sensor, can impair the quality of the MOF filter by promoting excessive oxide dissolution. Research has explored alternative MOF structures, demonstrating that the insertion of a pyrene core increases benzene adsorption and that palladium has a higher affinity for aromatic compounds than copper.

3. A Reliable Manufacturing Process (Area 3)

Another major success is the development of a reliable manufacturing method for sensor prototypes. The deposition of the sensitive GZO film is localized using a nickel metal mask (shadow mask), eliminating the need for photolithography. The mask, developed by electrochemical growth, allows for precise alignment of ±50 μm and ensures that the sensitive layer covers the entire surface of the electrodes. After annealing at 600°C, the chips are mounted on a T08 package and connected by wedge bonding (gold wires). This process combines sputter deposition and high-precision masking, enabling localized, controlled, and reproducible deposition of NO2 sensor prototypes.

Future Prospects: Towards Industrial Sensors for NO2 The SensMOFAir project opens up very promising prospects, particularly thanks to the remarkable results obtained with the ZnO:Ga-based sensitive layer.

Current Strengths: High Sensitivity and Natural Selectivity

• Exceptional Sensitivity: The sensitive layers demonstrated resistance variations of more than a decade for NO2 concentrations of around 100 ppb.

• Natural Selectivity: Contrary to the usual limitations of semiconductor oxide sensors, the developed ZnO:Ga layer demonstrated natural selectivity for NO2 over interferents such as NH3, benzene, or acetone.

This combination of high sensitivity and good natural selectivity is a major asset that makes it possible to consider integrating these sensors into industrial systems.

Monitoring of indoor and outdoor air pollution has become a key point in our everyday life. In particular, the European Commission calls on Member States to implement air quality action plans, which would ensure compliance with European directive standards. According to Markets and Markets, the global market for gas sensors, detectors and analyzers is expected to grow at an annual rate of 5.7% between 2016 and 2021 to reach $ 4 billion. Over the same period, the global market for electronic test and measurement instrumentation is expected to grow at an annual rate of 4.9%. Air quality management is currently based on a network of a small number of high precision fixed stations with very high purchase, operation and maintenance costs. Hence, there is currently a new paradigm for air pollution monitoring based on the use of ubiquitous network of low-cost sensors for real-time and high spatiotemporal resolution monitoring of air pollutants concentrations. The very few studies about the performance assessment of such low-cost sensors vs their more expensive counterparts show the sensors performance getting deteriorated under real-world conditions because of both ageing and influence of gaseous co-pollutants. In this respect, the aim of SensMOFAir-2020 project is to develop low-cost metal oxide gas sensors (MOS) for real-time and continuous measurement of air pollution, with high detection performance and low energy consumption. All together, these criteria will allow a massive, distributed and ubiquitous monitoring of ambient air quality. To overcome the major obstacle for a widespread use of MOS gas sensors (low selectivity), nanostructured sensitive layers developed and integrated on micromachined platforms will be further functionalized with selective molecular sieve filters improving sensors selectivity. Final products developed in this project (prototypes of selective gas sensors) will be tested in real environment with the involvement of the industrial partner. In the near future, such sensors could integrate omnipresent sensor networks and allow a dramatic increase in the volume of air quality data with high spatial and temporal resolution. Finally, these sensors are expected to i) supplement conventional air quality monitoring, ii) improve the link between pollutant exposure and human health, iii) help for emergency response management and last but not least iv) increase community's awareness and engagement towards air quality issues.