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 similar to 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.