Synthesis of single-crystal SnO2 nanowires for NOx gas sensors application

Single-crystal SnO2 nanowires (NWs) were successfully synthesized and characterized as sensing materials for long-term NOx stability detection in environmental monitoring. Reproducible and selective growths of the SnO2 NWs on a patterned, 5 nm-thick gold catalyst coated on a SiO2/Si wafer as substrate were conducted by evaporating SnO powder source at 960 °C in a mixture of argon/oxygen ambient gas (Ar: 50 sccm/O2: 0.5 sccm). The as-obtained products were characterized by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman scattering, and photoluminescence (PL). The SEM and HRTEM images revealed that the products are single-crystal SnO2 NWs with diameter and length ranges of 70 nm–150 nm and 10 μm–100 μm, respectively. The three observed Raman peaks at 476, 633, and 774 cm−1 indicated the typical rutile phase, which is in agreement with the XRD results. The NWs showed stable PL with an emission peak centered at around 620 nm at room-temperature, indicating the existence of oxygen vacancies in the NW samples. The electrical properties of synthesized SnO2 NWs sensor were also investigated and it exhibited a negative temperature coefficient of resistance in the measured range (300–525 K). The calculated activation energy Ec of SnO2 NWs was 0.186 eV. Moreover, the SnO2 NW sensors exhibited good response to NOx gas. The response of the sensors to 5 ppm NOx reached 105% at an operating temperature of 200 °C.