LEVANTO - Visible light activated gas sensors based on semiconductors: an operando investigation by DRIFT and confocal micro-Raman spectroscopy

Abstract:

Gas sensors play a crucial role in modern technology and are employed in a wide range of fields, spanning from environment to healthcare and industry. Commonly used chemiresistive gas sensors, due to the high operating temperature necessary to activate chemical reactions at the surface for the majority of semiconductors, do not completely match the requirements imposed by the current innovation paradigm, addressed to a safe, green and reduced power-consumption technology. Therefore, the search for diverse sensing materials which embrace novel activation mechanisms is of groundbreaking importance. The use of visible light to activate gas sensitivity in semiconductors is a further, fundamental step towards the design of gas sensors with low power consumption and improved selectivity.
In this respect, the LEVANTO project aims at the study and exploitation of the gas sensing mechanisms, which lead to response formation of visible light activated semiconductors.
In particular, among the wide palette of functional materials probed for gas sensing applications, the project will be focused on high quality nanocrystalline zinc selenide (ZnSe) and zinc telluride (ZnTe), still slightly explored as gas sensing materials, which feature an energy band gap suitable for visible light activation and high surface area to enhance the solid-gas interaction at the surface.
Fundamental phenomena behind the absorption spectrum, optical properties of defects/vacancies and transport parameters of the photoexcited carriers in semiconducting films, as well as innovative strategies to complement investigation methodologies currently used will be pursued.
Photoluminescence and photoconductivity measurements together with transient absorption spectroscopy will lead to a complete understanding of the optical and conductive properties of materials, together with an effective definition of illumination parameters tuning the compromise between sensitivity and maintenance of real measuring conditions.
The LEVANTO project will combine two unexplored operando spectroscopic systems for the study of photoactivated chemoresistive gas sensors: diffuse reflectance infrared (DRIFT) and micro-Raman spectroscopy. Such innovative synergic experimentation will provide for a confident route to actually identify gas sensing mechanisms, simultaneously performing a spectroscopic investigation on chemical reactions occuring at the surface during sensor operation under light activation while acquiring the resistance variation of the sensing films.
The vision of the project is to demonstrate the reliable operability at room temperature of photoactivated gas sensors with a deep understanding of the physical and chemical mechanisms acting at the surface. This can enable a viral overspreading of miniaturized devices in which sensing layers and light-emitting elements are embedded, e.g. for portable/wearable sensors.

Risultati attesi: 

The investigation on the light activation of semiconductor gas sensors proposed by the LEVANTO project plays an important role in modern materials science due to its interdisciplinary nature and the perspectives for practical application of the results. The deliverables (D) of the LEVANTO project lie in the definition of a well-established procedure to investigate the sensing mechanism of semiconductors (SC) photoactivated by visible light, paving the way for the exploitation of the potentialities of such an activation method. 
D1. High quality nanocrystalline semiconductor materials for visible light activation. We firstly expect to merge the well-established electrically conductive properties of ZnSe and ZnTe with their optical properties, which are inherently present in them but not still explored, in the case of ZnTe, or completely explored, in the case of ZnSe, for photoactivated gas sensing. The modulation of conducting properties by optical stimulation would primarily enhance the exploitation of potential properties of SCs under test and then would further strengthen their capabilities for gas sensing. 
D2. Effective SC optical properties and radiation parameters for photoactivation in the visible light range. The extensive optical characterization of both powders and films will guide the production of highly functional SC and then to effective photoactivated gas sensing devices. Moreover, these characterizations will support the identification of an established correlation between optical properties of SC and experimental irradiation parameters for their visible light activated gas sensitivity. 
D3. Validation of operando spectroscopic techniques for gas sensitivity photoactivation. The LEVANTO project will actively contribute to the development and validation of innovative methods for the study of sensing mechanisms under visible light activation. In general, DRIFT spectroscopy is still limited in its use in research on SC gas sensitivity, also in thermal activation mode. So far, to the best of our knowledge, the setup proposed in this project is the first that allows the experimentation of DRIFT spectroscopy for photoactivated gas sensing SC devices. At the same time, the implementation of the operando approach for both DRIFT and micro-Raman spectroscopy is completely new and makes these analysis methods significantly progressive. 
D4. Photoactivation addressed to reducing gases. It is well-known that most photoactivated sensors are more sensitive to oxidizing molecules, such as NO and O, and only few works reported photo-enhanced sensitivity to H. Starting from the preliminary works on ZnSe and ZnTe gas sensors developed in the last years, which demonstrated a sensitivity of these functional SC towards such interesting gases, i.e. SO and CO, an additional deliverable that the LEVANTO project would achieve is concrete effort to develop high performance detection also of reducing gases, such as organic compounds.