A dispersion-corrected DFT insight into the structural, electronic and CH4 adsorption properties of small tin-oxide clusters

Highlights: • The characteristics of Sn_xO_y clusters, before and after CH₄ adsorption are studied. • By cluster size increasing, ring-shape to cube-like structural crossover is occurred. • CH₄ molecule is physisorbed on tin-oxide clusters via Van der Waals interactions. • The cube-like clusters possess higher chemical reactivity than ring-shape ones. • By CH₄ adsorption, the conductivity of clusters increased due to energy-gap reduction. The main goal of this paper is to investigate the structural and electronic properties of small tin-oxide clusters, Sn_xO_y (x = 2–6), before and after methane (CH₄) physisorption. To this end, we employ the dispersion-corrected density functional theory (DFT-D2). At first, we demonstrate the relative binding stability of Sn_xO_y clusters with a fixed number of tin atoms via analysis of their binding energy per atom and second-order energy differences. Our results show that the highest relative stability occurs at y = 2, y = 3, y = 4, y = 6 and y = 8 for Sn₂O_y, Sn₃O_y, Sn₄O_y, Sn₅O_y and Sn₆O_y clusters, respectively. Furthermore, with the cluster size increasing, the ring-shape to cube-like structural crossover is occurred. The higher average coordination and total number of bonds in cube-like clusters make their interatomic bonds weaker and longer. On the other hand, the cube-like clusters, in comparison with ring-shape ones, possess higher chemical potential and lower chemical hardness that lead to their more chemical reactivity. We then proceed to study the CH₄ adsorption properties of the resulted most stable clusters. The lowest-energy structures of CH₄-adsorbed clusters have been found by comparing the adsorption energy of different configurations. It is found that Van der Waals interactions lead to exothermic physisorption of CH₄ on each tin-oxide cluster with the adsorption energy ranging from −101 to −191 meV. In addition, upon CH₄ adsorption on Sn_xO_y clusters, the energy gap is decreased which shows an enhancement in the conductivity of the clusters. As a result, the CH₄ physisorption ability of these small studied tin-oxide clusters provides their application feasibility as the low-temperature CH₄ gas sensors.