Thermophoresis is a common mechanism that can drive autonomous motion of Janus particles under the right environment. Despite recent efforts to investigate the mechanism underlying the self-propulsion of thermophoretic particles, the interaction of particles with the substrate underneath the particle has remained unclear. In this work, we explore the impact of poly(N-isopropylacrylamide) (PNIPAM)-functionalized substrate with various chain lengths on the active motion of a single polystyrene particle half-coated with gold (Au-PS). We show how the modification of the substrate with polymer brushes enhances the particle velocity, where brush chain length plays a significant role as well. The results demonstrate the intrinsic dependence of particle velocity on the flow boundary condition and the thermoosmotic slip at the interface.