Exploring formaldehyde gas sensing performance of NiO nanostructures: Experimental and theoretical perspectives

The research explores hydrothermally synthesized NiO nanostructures, including NiO-NA, NiO-HMDA, and NiO-HMTA, for formaldehyde gas sensing applications. XRD analysis confirmed a cubic crystal structure with space group Fm-3m, exhibiting well-defined and consistent arrangements. The Debye Scherer equation determined average crystallite sizes of 8.49 nm, 9.21 nm, and 8.42 nm for NiO-NA, NiO-HMDA, and NiO-HMTA, respectively. Distinct morphologies emerged, such as net-like structures for NiO-NA, altered nanosheets for NiO-HMDA, and feather-flower patterns for NiO-HMTA. Energy-dispersive X-ray spectroscopy affirmed the uniform distribution of Ni and O elements in all samples. UV–vis. spectra revealed absorption characteristics in the 370–390 nm range, with band gap values of 3.333 eV, 3.234 eV, and 3.243 eV for NiO-NA, NiO-HMDA, and NiO-HMTA, respectively. Gas sensing assessments demonstrated superior performance for NiO-HMTA at 350 °C, attributed to its smaller size and intricate morphology. Linear relationships between gas response and concentration were established, with NiO-HMTA exhibiting the highest precision. Further, rapid response (79 s) and recovery (84 s) times at 350 °C. The proposed mechanism highlights chemisorption-driven redox reactions, elucidating the intricate gas-sensing behavior of NiO-HMTA. The investigation into the interaction dynamics between formaldehyde (HCHO) and NiO was conducted through Density Functional Theory (DFT) simulations, employing the B3LYP/LanL2dz level of theory. The study discerned that the improved sensitivity of the NiO-based sensor to the HCHO molecule likely stems from the deposition of an oxygen atom from the surrounding medium onto the surface of NiO.