Fabrication and Characterization of IrO_x Nanosheets for Methane Sensors

<b>Introduction</b><br/>CH₄ is a greenhouse gas that throughs the second largest impact on global warming after CO₂, and it is essential to develop a technology to continuously monitor CH₄ emissions [1]. Recently, it has been reported that C-H bond of CH₄ can be cleaved at the IrO₂(110) surface in an ultrahigh vacuum [2]. However, the reaction has only been verified in ultrahigh vacuum, and it has not yet been clarified whether the C-H cleavage reaction by IrO₂ occurs in the standard atmosphere or whether this reaction can be applied to CH₄ resistive sensors.<br/>In this study, IrO_x nanosheets with IrO₂(110) were fabricated using reactive sputtering system and examined their resistive response to CH₄ in dry air. The impacts of film deposition conditions on the sensor response were also investigated.<br/><br/><b>Experimental Method</b><br/>To fabricate IrO_x-based CH₄ sensors, reactive sputtering system (Target: Iridium, Base pressure: 5×10^-3 Pa, RF power: 80 W, Substrate temperature: as-is) was used. IrO_x nanosheets of approximately 30 nm and 5 nm in thickness were deposited on 1-μm-thick thermal oxide (SiO₂) films on Si substrates. The chamber pressure during the deposition process was 0.67 Pa, and the flow ratio of O₂ against the total flow of 20 sccm was set to 10, 20, or 40%. After IrO_x deposition, the films were annealed process at 500°C for 30 min in an O₂ atmosphere. X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements were performed to 30-nm-thick IrO_x nanosheets. On the 5-nm-thick IrO_x nanosheets, 50-nm-thick Au electrodes were deposited, and the electrode pattern was formed by a metal shadow mask. The fabricated sensors were initialized by exposing to dry air for 5 minutes at 200°C. The sensing evaluation sequence is as follows. The sensors are 1) exposed to dry air with 50-ppm CH₄ for 1 minute, and then 2) exposed to dry air without CH₄ for 3 minutes. The sensor characteristics were evaluated at 200°C by repeating 1) and 2) twice. The sensor responses (Δ<i>R</i>/<i>R</i>₀) were evaluated as the ratio of resistance change (Δ<i>R</i> = <i>R</i>-<i>R</i>₀) due to the target gas (CH₄) with respect to the initial resistance (<i>R</i>₀), where <i>R</i>₀ is the initial sensor resistance.<br/><br/><b>Results and Discussion</b><br/>In XRD 2<i>θ</i>-<i>θ</i> measurements of IrO_x nanosheets after O₂ annealing, the peak from IrO₂ (110) was observed, regardless of the O₂ flow ratio. The broadening of the IrO₂(110) peak width indicates that the size of the IrO₂ microcrystals decreased with an increase in the O₂ flow ratio. XRR measurements of the nanosheets showed that the density of IrO_x nanosheets decreased as the O₂ flow ratio increased, implying that the nanosheets became sparser.<br/>Nanosheets made with a 10% O₂ flow ratio showed no sensor response at 200°C, whereas the sensor was responded to CH₄ at 200°C for the nanosheets made with 20 and 40% O₂ flow ratio, where the size of IrO₂ grains and density of IrO_x films were smaller than the 10% counterpart. This suggests that the size of IrO₂ crystals and/or the voids in the IrO_x nanosheets affected the sensor response to CH₄. In the previous study, it has been reported that CH₄ is oxidized by the bridging O atoms (O_br) on coordinatively unsaturated IrO₂ (110) surface and is desorbed as CO or CO₂ [3]. Thus, we consider that the oxidation reaction and resultant desorption of CH₄ and O_br led to the increase in resistance because the hopping transfer of electrons between the microcrystals was more difficult due to increased gaps between microcrystals. Therefore, it is expected that the sensor response to CH₄ can be increased by fabricating IrO_x nanosheets with narrower gaps between the microcrystals.<br/><br/><b>Acknowledgements</b><br/>This work was partly supported by JSPS KAKENHI Grant Number 18H05243, 19H00756 and JST CREST Grant Number JPMJCR1912.<br/><br/><b>References</b><br/>[1] T. Hong <i>et al.</i>, <i>Trends Analyt. Chem.</i>, <b>125</b>, 115820 (2020). [2] Z. Liang <i>et al.</i>, <i>Science</i>, <b>356</b>, 299 (2017).<br/>[3] C. J. Lee <i>et al.</i>, <i>J. Phys.: Condens. Matter</i>, <b>34</b>, 284002 (2022).