Hexagonal annular-NiO nanoarchitecture with local p-n homojunctions: novel formation mechanism and H2S gas sensing properties

This work reported a facile and scalable route that can control the synthesis of hexagonal annular-NiO nanoarchitecture (HANN) and explore its formation mechanism as well as gas-sensing properties. The results show that the morphology and crystal structure of HANN exhibit a uniform population with a fascinating annular shape, each containing an interior hexagonal hollow. The novel formation mechanism of HANN, which is closely related to the evolution of defects and nanopores during the annealing, is examined and discussed in detail by using nuclear analytical spectroscopies such as Positron annihilation lifetime, Doppler broadening and Electron momentum distribution. In particular, we found the possible formation of the so-called local p-n homojunctions due to the presence of O vacancies with a high concentration in the vacancy clusters in HANN. The high concentration of these vacancies generates many active sites and subsequently the local p-n homojunctions, which increases the adsorption/reaction of H2S molecules and the H2S-sensing performance of HANN. Hence, HANN-based sensors show excellent response, a low limit of detection (∼12 ppb) with remarkable selectivity and high stability toward H2S. This study is, thus, particularly important since it provides a new way to understand deeply the formation mechanism and gas-sensing performance of nanostructured metal oxide semiconductors.