In a recent article published in the journal Photonics, researchers investigated the impact of femtosecond (FS) laser irradiation on SnO2-nanowire gas sensors for toluene gas sensing.
Schematic diagram of sensing mechanism of the laser-irradiated SnO2 NWs in terms of oxygen vacancy and electron depletion. Image Credit: https://www.mdpi.com/2304-6732/11/6/550
Background
Gas sensors using semiconductor metal oxides detect and monitor various gases, safeguarding the environment, human health, and industrial processes. Toluene (C7H8), a volatile organic compound commonly found in household products, poses health risks upon prolonged exposure, highlighting the importance of accurate and efficient gas sensors for detecting such compounds.
To improve gas-sensing capabilities, researchers have explored various strategies, including heterojunction formation, noble-metal functionalization, doping, surface engineering, and irradiation techniques. Surface engineering aims to enhance sensing performance by increasing surface area using nanostructured materials or surface roughness modifications. Irradiation techniques, such as laser irradiation, show promise in improving gas sensor performance by inducing structural modifications and surface alterations in the sensing material.
The Current Study
The Au-catalyzed vapor–liquid–solid (VLS) technique was employed for the synthesis of SnO2 nanowires. A Si wafer with a SiO2 layer served as the substrate, onto which tri-layered electrodes (Ti, Pt, Au) were deposited. Metallic Sn powder was placed on the tri-layer-deposited substrate in a quartz tube furnace under a controlled gas flow of N2 and O2. The furnace temperature was maintained at 900 °C to facilitate the growth of SnO2 nanowires on the substrate.
The experimental setup for femtosecond laser irradiation involved the use of a laser with specific parameters: central wavelength of 1030 nm, pulse duration of 350 fs, and frequency of 15 kHz. The laser beam was focused on the gas sensor device with a beam diameter of 4.8 μm. The laser beam scanned at a speed of 80 mm/second.
Qualitative analysis of the SnO2 nanowires was conducted using X-Ray diffraction (XRD) with Cu-Kα radiation. Morphology and microstructure examination were performed via field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The fabricated SnO2 nanowires were dispersed in a solvent and deposited onto a TEM grid for microscopy. Electron energy loss spectroscopy (EELS) was utilized to examine the surface states of the pristine and laser-irradiated SnO2 nanowires.
A customized gas-sensing system was employed to evaluate the gas-sensing behavior of the SnO2 nanowires. A gas chamber placed in a quartz tube furnace allowed for controlled operating temperatures. The pressure of the target gas, C7H8, was carefully regulated during the gas-sensing experiments to assess the response of the gas sensor to varying gas concentrations.
Results and Discussion
The Au-catalyzed VLS method successfully synthesized SnO2 nanowires on the substrate, confirmed by XRD analysis showing consistent patterns with crystalline SnO2. FE-SEM and TEM analyses revealed the morphology and microstructure of the nanowires. TEM images displayed a smooth and flat surface for pristine SnO2 nanowires, while laser-irradiated nanowires at 138 mJ/cm² exhibited nano-sized bumps on the surface, indicating structural changes induced by the femtosecond laser.
EELS analysis was conducted to examine the chemical characteristics of the pristine and laser-irradiated SnO2 nanowires. The results revealed the formation of SnO and SnOx phases in the laser-irradiated nanowires, indicating a change in the chemical composition due to the laser irradiation process. This alteration in chemical composition could influence the nanowires' gas-sensing behavior.
The gas-sensing behavior of the laser-irradiated SnO2 nanowires was superior to that of the pristine nanowires, with enhanced sensor response levels observed. Specifically, the sensor response level at 138 mJ/cm² laser energy density surpassed that of the pristine nanowires, indicating improved gas detection capabilities. The embossed surface created by laser irradiation and the generation of oxygen deficiency in the nanowires were identified as key factors contributing to the enhanced gas-sensing performance.
Conclusion
In conclusion, the results of this study highlight the effectiveness of femtosecond laser irradiation in improving the gas-sensing capabilities of SnO2 nanowires. The structural modifications induced by the laser treatment, along with the chemical changes observed, offer valuable insights for enhancing gas sensor performance. The findings pave the way for the development of more efficient and reliable gas sensing technologies, with potential applications in various industries requiring sensitive and selective gas detection systems.
Journal Reference
Ahn, S.; Chun, K.W.; Park, C. Long-Term Stability Test for Femtosecond Laser-Irradiated SnO2-Nanowire Gas Sensor for C7H8 Gas Sensing. Photonics 2024, 11, 550. https://doi.org/10.3390/photonics11060550, https://www.mdpi.com/2304-6732/11/6/550