Adsorption of toxic SO_x molecules on heterostructured TiO₂/ZnO nanocomposites for gas sensing applications: a DFT study

Using density functional theory (DFT) calculations, we predict the SO_x sensing performance of heterostructured TiO₂/ZnO nanocomposites with and without nitrogen doping. The interaction of SO₂ and SO₃ molecules with the considered nanocomposites were examined based on different orientations of the gas molecules towards the nanocomposite. The fivefold coordinated titanium atoms were found to be the binding sites on the TiO₂ side of nanocomposite, whereas, on the ZnO side, the oxygen atom acts as a binding site. Our theoretical results demonstrate that the interaction of SO_x molecules with N-doped nanocomposites is more energetically favorable than that with undoped ones, indicating that N-doped TiO₂/ZnO nanocomposites show stronger chemisorption and greater electron transfer effects than undoped TiO₂/ZnO. The electronic properties of the adsorption systems were investigated in terms of the projected density of states and molecular orbitals. After the adsorption process, all S–O bonds of the SO_x molecules were elongated, which is probably attributed the electron density transfer from the S–O bonds to the newly formed bonds between the nanocomposite and SO_x molecules. The charge transfer analysis revealed that N-doped nanocomposite acts as a donor. The N-doped nanocomposite induce dramatic changes of electronic properties of TiO₂/ZnO, which can be useful feature for improving the gas sensing performance. Our calculation results aim to provide some information for future experiment.