Metal oxide semiconductor-based core-shell nanostructures for chemiresistive gas sensing: A review

Metal oxide semiconductor (MOS)-based core-shell nanostructures with the unique structural and synthetic superiority have been universally applied for the chemiresistive gas sensors. We summarize the recent work involving MOS@MOS and noble metal@MOS core-shell nanostructures and further relate the improved performance to the work function-induced electron transfer through the N-N, N-P or Schottky junction. Generally, the performance can be enhanced if electrons transfer from the shell to the core and the heterostructure shows the same N- or P- type behavior to the shell MOS. Besides, the shell thickness is regarded to play a significant role in the performance modulation including the N-to-P (or P-to-N) conversion. The utilization of noble metals can sometimes change energy barriers and reaction routes of gas sensing, especially the adoption of Pd in detecting H2. Moreover, alloying which can prevent the oxidation of noble metals, and constructing more complicated multi-shell nanostructures are both discussed as the potential development directions in this field.