In the present work, we investigate the influence of hydrocarbon (CH x ) thin film coating on porous silicon (PS) by plasma enhanced chemical vapor deposition (PECVD) technique to detect CO 2 gas. The fabricated CH x /PS heterojunction device shows up to one and two orders of magnitude enhancement in current under CO 2 gas exposure. FTIR spectroscopy measurements reveal a remarkable structural modification of the CH x /PS device during CO 2 gas exposure. Further, the enhancement of CH x related absorbance bands by a factor 6.2 for the CH x /PS specimen in comparison with PS confirm the good quality of the deposited CH x thin films.