Nowadays, the development of innovative and low-cost smart gas sensors is required in many applications, including medical screening, environmental monitoring and precision farming. Chemoresistive gas sensors are the most widely studied solid state gas sensors in this perspective, due to their small size, low production cost and high sensitivity [1]. However, the lack of selectivity of the nanostructured metal oxides (MOX), i.e. the most widely used class of sensing material so far, limited the effective and widespread adoption of these devices in many applications. In the last few years, great attention has been paid on the development of innovative sensing materials with advanced chemoresistive properties, able to overcome the shortcomings of the typical MOX, seeking the optimization of the sensing performance. Modified MOX (doped or functionalised) proved to be good candidates, owing to the right combination of typical MOX stability and improved selectivity due to surface sensitisation [2]. Considering doping, the investigation of the influence of intrinsic dopants on the MOX sensing properties has attracted considerable attention recently, particularly with regard to oxygen vacancies (Ov), which have shown to have huge impact on the MOX electrical properties and surface reactivity. In this work, a specific reducing treatment at high temperature has been investigated in order to develop reduced WO3 with controlled Ov concentration. Nanostructured WO3 has been synthesised by using a simple sol gel method. Then, a calcination treatment at 650ºC in air has been carried out in order to obtain a nanocrystalline and stoichiometric WO3. A rapid thermal annealer (RTP) has been employed for the controlled reduction of the WO3 nanoparticles, by using H2 (4% in N2) as reducing agent. Different times (15 and 30 minutes) and temperatures (from 300 to 800ºC) were investigated, in order to study their impact on the Ov formation. The Ov in the reduced samples were characterized by using SEM-EDX, XRD and XPS. The XPS characterization has revealed a strong increase in the 5+, 4+ and 3+ oxidation states of W as the treatment temperature rises, due to a strong increase in the surface concentration of Ov. Both in-plane and bridging Ov were formed. An increase of the bulk Ov has been observed as well by XRD analysis. On the other hand, the concentration of Ov did not change significantly with treatment time. The reduced powders were deposited on silicon substrates and their sensing performances were investigated vs. NH3. WO3 reduced at 700ºC showed the best sensing performance towards NH3 at near room working temperature, showing an impressive increase of the sensitivity and selectivity compared to stoichiometric WO3. The role of surface Ov in the sensing mechanism is under investigation.
Investigation on oxygen vacancies influence on reduced WO3 sensing properties
Andrea Gaiardo
;Lia Emanuela Vanzetti
2022-01-01
Abstract
Nowadays, the development of innovative and low-cost smart gas sensors is required in many applications, including medical screening, environmental monitoring and precision farming. Chemoresistive gas sensors are the most widely studied solid state gas sensors in this perspective, due to their small size, low production cost and high sensitivity [1]. However, the lack of selectivity of the nanostructured metal oxides (MOX), i.e. the most widely used class of sensing material so far, limited the effective and widespread adoption of these devices in many applications. In the last few years, great attention has been paid on the development of innovative sensing materials with advanced chemoresistive properties, able to overcome the shortcomings of the typical MOX, seeking the optimization of the sensing performance. Modified MOX (doped or functionalised) proved to be good candidates, owing to the right combination of typical MOX stability and improved selectivity due to surface sensitisation [2]. Considering doping, the investigation of the influence of intrinsic dopants on the MOX sensing properties has attracted considerable attention recently, particularly with regard to oxygen vacancies (Ov), which have shown to have huge impact on the MOX electrical properties and surface reactivity. In this work, a specific reducing treatment at high temperature has been investigated in order to develop reduced WO3 with controlled Ov concentration. Nanostructured WO3 has been synthesised by using a simple sol gel method. Then, a calcination treatment at 650ºC in air has been carried out in order to obtain a nanocrystalline and stoichiometric WO3. A rapid thermal annealer (RTP) has been employed for the controlled reduction of the WO3 nanoparticles, by using H2 (4% in N2) as reducing agent. Different times (15 and 30 minutes) and temperatures (from 300 to 800ºC) were investigated, in order to study their impact on the Ov formation. The Ov in the reduced samples were characterized by using SEM-EDX, XRD and XPS. The XPS characterization has revealed a strong increase in the 5+, 4+ and 3+ oxidation states of W as the treatment temperature rises, due to a strong increase in the surface concentration of Ov. Both in-plane and bridging Ov were formed. An increase of the bulk Ov has been observed as well by XRD analysis. On the other hand, the concentration of Ov did not change significantly with treatment time. The reduced powders were deposited on silicon substrates and their sensing performances were investigated vs. NH3. WO3 reduced at 700ºC showed the best sensing performance towards NH3 at near room working temperature, showing an impressive increase of the sensitivity and selectivity compared to stoichiometric WO3. The role of surface Ov in the sensing mechanism is under investigation.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
