Revealing Gas Sensing Mechanism of PtO₂ Originated from The Structural Equivalence with SnO₂

Conventional gas sensing with Pt relies on the spillover effect, where Pt nanoparticles split gas molecules and disperse onto n-type semiconductors such as ZnO, WO₃, and SnO₂. In the presence of PtO, the p-type semiconductor nature of PtO creates an electron depletion region at the heterojunction with n-type semiconductors, which in turn affects the gas sensor resistance and thus provides information about the surrounding gas composition. The gas sensing mechanism of another oxidation state, PtO₂, has conventionally been considered a p-type semiconductor, similar to PtO until now. However, this traditional interpretation does not explain the increased selectivity and sensitivity observed with PtO₂ compared to both PtO and Pt. There is also a lack of research comparing the gas sensing properties based on the different oxidation states of Pt.<br/>In this study, we suggest a novel gas sensing mechanism of PtO₂when combined with SnO₂, originating from the structural equivalency between PtO₂ and SnO₂. Previously, it has been demonstrated that at elevated temperatures and under high O₂ partial pressure conditions, Pt in PtO₂ can replace Sn in SnO₂ lattice due to their identical rutile structure and oxidation state (4⁺) as well as their similar ionic radii. Additionally, PtO₂, known for its strong oxidizing properties, creates oxygen vacancies in a reducing gas atmosphere. These vacancies then become active sites for the adsorption of target gases, significantly enhancing gas sensor performance. To verify the sole influence of the oxidation state of Pt, we designed 3D-aligned Pt decorated SnO₂ nanowires using Thermally-assisted nanotransfer printing(T-nTP). This method improved mass transport, ensured consistent network connections, and increased the surface area, enabling specific focus on the differences originating from the oxidation state of Pt. Via annealing in an air furnace, the oxidation states of Pt were selectively altered while keeping the other conditions, such as the structure of the gas sensor and the state of the supporting material, SnO₂. Specifically, annealing the gas sensor at 900°C results in approximately 99% conversion of Pt into PtO₂. We observed that PtO₂ decoration on SnO₂ exhibited superior sensitivity and selectivity for hydrogen sulfide (H₂S) compared to Pt decoration. PtO₂ showed approximately 60 times greater sensitivity compared to the sample annealed at a lower temperature, achieving response ratios (R_air / R_gas) exceeding 50 at a concentration of 1 ppm. Overall, this study presents a newly discovered gas sensing mechanism with PtO₂ and demonstrates significantly improved sensitivity and selectivity, particularly in detecting H₂S. Moreover, the gas sensors synthesized via T-nTP offer the potential for comparative analysis of gas sensing properties across different oxidation states of other noble metals.