Multifunctional Materials: A Case Study of the Effects of Metal Doping on ZnO Tetrapods with Bismuth and Tin Oxides

Hybrid metal oxide nano- and microstructures exhibit novel properties, which make them promising candidates for a wide range of applications, including gas sensing. In this work, the characteristics of the hybrid ZnO-Bi2O3 and ZnO-Zn2SnO4 tetrapod (T) networks are investigated in detail. The gas sensing studies reveal improved performance of the hybrid networks compared to pure ZnO-T networks. For the ZnO-T-Bi2O3 networks, an enhancement in H-2 gas response is obtained, although the observed p-type sensing behavior is attributed to the formed junctions between the arms of ZnO-T covered with Bi2O3 and the modulation of the regions where holes accumulate under exposure to H-2 gas. In ZnO-T-Zn2SnO4 networks, a change in selectivity to CO gas with high response is noted. The devices based on individual ZnO-T-Bi2O3 and ZnO-T-Zn2SnO4 structures showed an enhanced H-2 gas response, which is explained on the basis of interactions (electronic sensitization) between the ZnO-T arm and Bi2O3 shell layer and single Schottky contact structure, respectively. Density functional theory-based calculations provide mechanistic insights into the interaction of H-2 and CO gas molecules with Bi-and Sn-doped ZnO(0001) surfaces, revealing changes in the Fermi energies, as well as charge transfer between the molecules and surface species, which facilitate gas sensing.