CO₂ valorization has been recognized as a pivotal process in reducing carbon sequestration costs. While metal–organic frameworks (MOFs) exhibit remarkable CO₂ capture performance, their photocatalytic CO₂ reduction activity is often constrained by factors, such as insufficient active sites and frequent charge carrier recombination. In this work, an innovative strategy tailoring MOF surface architecture was developed. By tuning the concentration of highly supersaturated synthesis solutions, nonclassical secondary crystallization was facilitated, constructing abundant surface protrusions on the classic UiO-66 for the first time. These ultrasmall crystalline structures featuring high-index facets and missing-linker defects markedly increased both the amount and Lewis acid strength of exposed Zr sites, enhancing CO₂ adsorption capacity and kinetics. Concurrently, the surface architecture induced an interfacial homojunction with a supporting built-in electric field, precisely delivering the photogenerated electrons to adsorbed CO₂. Collectively, these upgrades synergistically boosted the critical surface reactions of the engineered catalyst, resulting in a sixfold increase in its CO yield relative to the pristine seeds. With its versatility, this strategy enriches the methodologies for addressing the intrinsic limitations of MOFs and enables the a priori design of efficient photocatalysts for low-concentration CO₂.