Debye-length controlled gas sensing performances in NiO@ZnO p-n junctional core-shell nanotubes

To enhance gas sensing performance, constructing p-n heterostructures are considered as a promising route for designing high-performance gas sensing materials due to their synergistic and p-n junction effects. In particular, the shell layers' thickness often plays a vital role in the modulation of carrier transport during the gas sensing processes. In this work, we have designed a type of NiO@ZnO core-shell nanotube (CSNT) with a thickness-tuneable shell by combining electrospinning with atomic layer deposition (ALD) techniques. The results showed that the NiO@ZnO composite nanofibers possessed a uniform tubular structure and comprised of a 230 nm polycrystalline NiO core and a wrinkled porous ZnO shell with a tuneable thickness (0-50 nm) via ALD cycles. Also, gas sensing tests showed that the NiO@ZnO CSNTs with shell thickness close to the Debye length showed the highest gas sensitivity, e.g. the response to 100 ppm ethanol was 15.8, which is ∼6.5 times that of the pure NiO. Moreover, the assembled sensors also showed excellent stability (almost keeping 100% after 1 month tests), increased response speed and improved gas selectivity. Furthermore, based on our series of tests and analysis, a possible gas sensing enhancement mechanism (Debye-length controlled gas sensing mechanism) has been proposed for the gas sensing behaviours of our designed sensors based on NiO@ZnO CSNTs.