Novel CuCo2O4 photonic crystals for optical hydrogen sensing: catalyst-free detection and mechanistic insights via in situ Raman spectroscopy†

Hydrogen is emerging as a promising fuel source for a sustainable, carbon-free future. However, its explosive nature necessitates robust safety measures, including optical hydrogen sensors, ideal for detecting minor leaks in hazardous environments due to their non-contact operation. In this study, we investigate two photonic crystal structures using a novel transition metal oxide CuCo₂O₄ (CCO) – CCO opal and inverse opal – for sensing hydrogen through dynamic reflectance measurements. CCO opal shows a detection range from 25% to 1% with the decrease in the intensity of the photonic band gap with a shift of 5 nm with a response time of 20 minutes, while CCO inverse opal detects 1–0.3% of hydrogen with a 12 nm shift within 2 minutes with a change in the effective refractive index of 0.0159. Furthermore, in situ Raman spectroscopic studies reveal that the change in the vibrational modes of CCO on exposure to hydrogen results in the formation of an intermediate compound structurally analogous to CoO, causing a change in the effective refractive index. The metal ion coordination shows distinct changes favoring a tetrahedral environment for reduced metal ions. The ratio of the intensity of A_1g mode and F_2g mode shows a decrement with time when exposed to 5–0.5% of hydrogen with a response time of 3 minutes, which coincides with the optical sensing response. Thus, the optical gas sensors are fabricated using scalable and facile techniques for the detection of hydrogen without any noble metal catalyst and demonstrating room temperature application for safety and process control.