Recent research has demonstrated that heterogeneous charge-transfer reactions are not restricted to conductors and that electrochemical reactions can occur on the surface of statically charged insulators. However, the exact mechanism by which insulators gain and lose electrical charges remains controversial. Herein we have studied quantitatively the reduction of silver ions on intrinsic amorphous silicon surfaces that are statically charged by contact against plastic polymers. We have quantified the magnitude of the redox work done by the tribocharged silicon surface as a function of its adhesion and hardness, with these two variables being tuned using covalent Si—C monolayer chemistries. We observed that metallic particles grow preferentially over surfaces that are relatively soft (low DMT modulus) and highly adhesive, hence indirectly proving that the triboelectrification of an insulator–insulator dynamic contact is caused by the exchange of ionic fragments, rather than by the movement of free electrons. This work clarifies the origin of triboelectricity, devises a surface-chemistry method to maximize tribocharging with immediate scope in single-electrode electrochemistry, and describes a concept potentially suitable for the mask-free and bias-free patterning of metal nanoparticles on photoconductors.