Room-Temperature Gas Adsorption on Standing and Zigzag ZnO Nanowires: A ReaxFF-MD Study

Gas adsorption is intrinsically governed by the surface structure of sensing materials, where a large surface area and a high reactivity are critical for optimal performance. Nanostructured materials, with their exceptional surface-to-volume ratios, offer enhanced sensing capabilities. In this study, we designed two distinct ZnO nanowire morphologies (standing and zigzag) and used reactive force field molecular dynamics (ReaxFF-MD) simulations to evaluate their adsorption performance for O₂, H₂, Cl₂, CH₂O, C₂H₆O, and C₃H₆O at 300 K with an initial gas level of 200 molecules per system. The results show that all molecules fully adsorb on the ZnO nanowire surfaces, O₂ dissociates into O atoms, and C₂H₆O dissociates into C₂H₅O and H atoms, both attached to the surface. Energy analysis shows that standing nanowires exhibit more negative adsorption energies for nonpolar gases (O₂, -10317.26 kcal/mol; H₂, -385.67 kcal/mol; Cl₂, -19000 kcal/mol), whereas zigzag nanowires favor polar molecules (CH₂O, -9380 kcal/mol; C₂H₆O, -8568.58 kcal/mol; C₃H₆O, -6897.73 kcal/mol). Both morphologies display a two-stage adsorption process, with adsorption numbers in the second stage decreasing in the following order: O₂ > C₂H₆O > CH₂O > C₃H₆O > Cl₂ > H₂. Compared to zigzag nanowires, standing nanowires adsorb 136 O₂, 9 H₂, 15 Cl₂, 38 CH₂O, 53 C₂H₆O, and 29 C₃H₆O molecules, whereas zigzag nanowires adsorb 129 O₂, 10 H₂, 18 Cl₂, 37 CH₂O, 44 C₂H₆O, and 27 C₃H₆O molecules. In both morphologies, O₂ adsorption leads to a strong improvement in sensitivity. These results pave the way for smarter nanoscale designs in next-generation gas sensors.