The research and development of high-performance NH 3 sensors are of great significance for environment monitoring and disease diagnosis applications. Two-dimensional (2D) MoS 2 nanomaterials have exhibited great potential for building room-temperature (RT) NH 3 sensors but still suffer from relatively low sensitivity. Herein, the TiO 2 -modified monolayer MoS 2 films with controllable TiO 2 loading contents are fabricated by a facile approach. A remarkable enhancement in the RT NH 3 sensing performance is achieved after the n - n hetero-compositing of the TiO 2 /MoS 2 system. The device with 95% surface coverage of TiO 2 shows enhanced sensor response, low detection limit (0.5 ppm), wide detection range (0.5-1000 ppm), good repeatability, and superior selectivity against other gases. In situ Kelvin potential force microscopy results revealed that the TiO 2 modification not only improved the surface reactivity of the sensing layers but also contributed to the NH 3 sensing performance by serving as the "gas-gating" layers that modulated the electron depletion layer and the conductivity of the MoS 2 films. Such an n - n hetero-compositing strategy can provide a simple and cost-effective approach for developing high-performance NH 3 sensors based on 2D semiconductors.