Table 1.

A summary about factors affecting gas sensing performances of ZnO hierarchical structure based gas sensors and improvement approaches.

Main characteristic indexes which reflect the performances	Influencing factors and improvement approaches	References
Sensitivity	Modulation of the dimensional and the exposed crystal facet of their constituting building blocks	Zhang et al., 2009
	Enhance the porosity of hierarchical structures	Lei et al., 2017; Song et al., 2018
	Modification by doping with noble metals and loading other n-type or p-type MOS materials	Lin et al., 2015
	Control of grain size	Mirzaei et al., 2018
Selectivity	Dope with noble metals and p-type metal oxides	Li T. M. et al., 2015
	Lower the operating temperature by activating the sensing material under UV illumination	Chen Y. et al., 2016; Espid and Taghipour, 2017
Long-term stability	Calcination/annealing as the post-processing treatment	Gu et al., 2011
	Reduce the working temperature of gas sensing element	Chen Y. et al., 2016
	Dope noble metal or synthesis of mixed oxides	Dey, 2018