The effect of surface modification and catalytic metal contact on methane sensing performance of nano-ZnO–Si heterojunction sensor

A sol–gel derived Nanocrystalline Nanoporous ZnO–Si heterojunction device was fabricated. Two types of heterojunction structure, one with as deposited ZnO film and another with surface modification (by palladium nanoclusters), via a simple aqueous route, were investigated for methane sensing. Three configurations with different contact metals (Pd–Ag contact on both sides/Pd–Ag on ZnO side and Au on Si / and Au on both sides of the junction) were studied. The heterojunctions were investigated at different operating temperatures (50–300 °C) and at different operating voltages (1–5 V) for varying concentrations of methane (0.01%, 0.05%, 0.1%, 0.5%, 1.0% and 1.5%). The surface modified sensors offered much lower operating temperature than that of unmodified ones. It was observed that the Pd modified device with Pd–Ag (70%) contacts on both sides offered faster response (∼23 s) with a higher response magnitude (∼73%) compared to the unmodified sensor of similar configuration (response time ∼28 s and response magnitude ∼63%). It was further revealed that the Pd surface modification has a more significant effect on the devices with noncatalytic contact compared to their counterpart with catalytic contact.