P-type metal oxide semiconductor with extraordinary surface reactions is highly active for gas sensing reactions. However, presenting large active sites in materials surface for sensing reactions is a great challenge. Here, we report a strategy to fabricate Co₃O₄ hexagonal platelets and characterize the adsorption of ammonia (NH₃) by using density-functional theory (DFT) calculations. The as-prepared Co₃O₄ hexagonal platelets exposed by (112) crystal plane shows high selectivity and sensitivity to NH₃ at room temperatures. In addition, fantastic reproducibility of the device is also demonstrated. Such alluring sensing performances can be attributed to unique size, structure, and surface arrangement on the Co₃O₄ platelets that facilitate the NH₃ adsorption and interface interaction. The present studies provided a new insight into improving the selectivity of gas sensors by designing special crystal-facets exposure in the nanomaterials surface matrix.