Network structures assembled from alpha-Fe2O3 nanosheets with exposed {104} facets were successfully prepared by heating Fe(NO3)(3) solution containing polyvinylpyrrolidone (PVP) in air. The alpha-Fe2O3 nanosheet-based network structures demonstrate significantly higher response to ethanol and triethylamine than alpha-Fe2O3 commercial powders. The excellent sensing performances can be ascribed to the exposed (104) facet terminated with Fe atoms. A concept of the unsaturated Fe atoms serving as the sensing reaction active sites is thus proposed, and the sensing reaction mechanism is described at the atomic and molecular level for the first time in detail. The concept of the surface metal atoms with dangling bonds serving as active sites can deepen understanding of the sensing and other catalytic reaction mechanisms and provides new insight into the design and fabrication of highly efficient sensing materials, catalysts, and photoelectronic devices.