Gas sensors play a significant role in the fields of environmental monitoring

industrial production

safety

medical diagnosis

military and aerospace. Solid state gas sensors possess advantages of small size

low power

high sensitivity and low cost. They can detect very low concentrations of a wide range of gases in the range of parts-per-million (mg·L-1). Graphene is a promising material for solid state gas sensors with high sensitivity due to its high carrier mobility

maximum surface-to-volume ratio

and low noise level. In this work

nano-grain graphene is grown on sapphire substrate by plasma enhanced chemical vapor deposition at a relative low temperature at 950℃ and a short time of 15 min. The as-prepared graphene shows bilayer structure with large amount of defects

which brings advantages for graphene gas sensors. The nano-grain graphene gas sensor on sapphire shows good response and recovery characteristics for formaldehyde gas

and exhibits good sensitivity of 3% response toward a concentration of formaldehyde gas of 20 mg·L-1 at room temperature with a response time of 9.92 min. The recovery time of the nano-grain graphene sensor for formaldehyde gas of 20 mg·L-1 is 17 min. Low energy barrier of the gas adsorption and desorption induced by large numbers of grain boundaries and wrinkles in the nano-grain graphene is the main reason for its good gas sense characteristics.