In order to exploit the potential applications of graphene as gas sensors, the adsorptions of a series of small gas molecules (such as CO, O_2, NO_2 and H_2O) on pristine graphene (PG) and Si-doped graphene (SiG) have been investigated by ab initio calculations. Our results indicate that the electronic properties of PG are sensitive to O_2 and NO_2 molecules, but not changed much by the adsorption of CO and H_2O molecules. Compared with PG, SiG is much more reactive in the adsorption of CO, O_2, NO_2 and H_2O. The strong interactions between SiG and the adsorbed molecules induce dramatic changes to the electronic properties of SiG. Therefore, we suggest that SiG could be a good gas sensor for CO, O_2, NO_2 and H_2O.
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