Homogeneous doping can boost solar-to-hydrogen conversion and therefore attracts great attention. Although a great deal of effort has been made to explore the doping-photoreactivity relationship, the doping mechanisms, especially from the perspective of crystal facets, are seldom explored. In this study, a general homogeneous carbon doping strategy is established and then serves as the doping model for a mechanistic investigation, as encouraged by its versatility in enabling homogeneous incorporation of carbon and improving solar-to-hydrogen conversion for typical oxides including TiO2, ZnO, and BiOCl. Using well-defined BiOCl nanosheets of high {001} or {010} facet exposure, we clarify the homogeneous carbon doping mechanism at the level of crystal facets for the first time. This mechanism involves the initial facet-dependent adsorption of the dopant precursor, regulated by the surface atomic structures, and the subsequent facet-dependent diffusion of carbon dopants associated with the facet-related arrangements of bulk atoms. This results in facet-dependent carbon doping behavior and a dopant-concentration-dependent solar-to-hydrogen conversion property of BiOCl nanosheets. These mechanistic insights also suggest that the implantation of the dopant precursor in the shallow lattice of host nanocrystal is vital for the effective homogeneous doping. This new doping model is different from the conventional counterpart based on the organic ligands or gas molecules adsorption onto the surface of host nanocrystals, where surface doping usually occurs.