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The greenhouse effect is involving global heating and climate change within the world. Carbon dioxide (CO2) is one of the major gas at the origin of this effect, but also the byproduct of human activity. Therefore, monitoring the indoor/outdoor CO2 emission by gas sensors is one of the priorities for environmental preservation. In this paper, the sensing performance of CO2 towards two different O-rich films have been studied; graphene oxide (GO) and vanadium dioxide (VO2). The preparation of GO film has been carried out by spray pyrolysis on fluorine tin oxide (FTO) prepared by the modified Hummers method. While the VO2 film has been sol-gel spin-coated on a glass substrate. Both films have been characterized using XRD, SEM and electrical properties. The CO2 gas sensing mechanism and the role of oxygen vacancies in VO2 are addressing. The oxygen functional groups in GO play a main role in the CO2 gas the sensitivity level and response time. Their gas sensing performances have been investigated based on measuring the response vs recovery time, dynamic response curve analysis and sensitivity. In order to better understand the sensing mechanism, characterization has been done with different gas concentrations. Both GO and VO2 based CO2-sensors are acted as an n-type sensor. Sensing behavior of GO at RT has explained to be mainly mediated by the oxygen functional groups and a wide range of active sites. In the other hand, VO2 contains oxygen vacancies and more defect sites which play a main role in the RT sensing activity and low recovery time. (C) 2019 Elsevier B.V. All rights reserved.