TiO2-Na0.9Mg0.45Ti3.55O8 (TiO2-NMTO) nanocomposites were synthesized via a simple hydrothermal method. TiO2 nanoparticles were loaded on NMTO nanosheets with well matched lattices. The TiO2-NMTO nanoheterojunctions enjoyed high photodegradative ability for a RhB pollutant. The photoinduced electron-hole pairs were separated effectively by the TiO2-NMTO nanoheterojunctions, which were directly observed by surface potential measurements with a scanning Kelvin probe microscopy. The photogenerated electrons accumulate at interface due to the high density of interface states, and holes remain TiO2 and NMTO particles, other than they migrate from one part to another in heterojunctions by comparing the surface potentials under illumination with different wavelengths.