Tetrahedral CuI crystals with broadly tunable sizes from 56 to 645 nm have been synthesized by adjusting the reaction temperature, addition of different amounts of Na2SO4, or the reagent selection. The crystal growth process can be easily scaled up for catalysis applications. Bulk and surface compositions have been determined to be CuI using various characterization methods. Although larger tetrahedra show gradually decreasing band gap values, the changes are quite small. Optical facet effect should also be present for CuI by comparing the band gaps of size-tunable CuI tetrahedra to that of commercially available CuI powder. Emission band also becomes red-shifted for larger tetrahedral samples. The 56 nm CuI tetrahedra exhibit a far better catalytic performance than that of commercially available CuI powder toward click reactions of aromatic alkynes and benzyl azide for the formation of triazoles, attributed to their large surface area and exposed {111} faces. The small CuI tetrahedral nanocrystals should be explored for use in other CuI-catalyzed reactions. (C) 2021 Elsevier Inc. All rights reserved.