Simultaneous control of the size and chemical composition is an advantageous strategy to obtain the desired photochemical properties of multinary semiconductor nanocrystals, ZnS-AgInS2 solid solution ((AgIn) x Zn2(1–x)S2, ZAIS) nanocrystals (NCs), being different from conventional binary nanocrystals. The energy gap (E g) of ZAIS NCs was enlarged with a decrease in particle size due to the quantum size effect or with an increase in ZnS content in the ZAIS solid solution. The levels of the conduction band edge and valence band edge, determined by photoelectron spectroscopy in air, were shifted more negatively and more positively, respectively, with an increase in E g. A volcano-type dependence was observed between the PL quantum yield (QY) and the size of ZAIS NCs, in which the optimal PL QY for each x value was obtained at a similar particle size around 5–6 nm, except for x = 1.0, and maximum QY was recorded to be 79% for ZAIS NCs prepared with x = 0.5. The photocatalytic activity for H2 evolution was also greatly dependent on both the size and the chemical composition of ZAIS NCs, and then the highest activity was observed for ZAIS NCs having an average diameter of about 4.2–5.5 nm and E g of 2.3–2.4 eV. This can be reasonably explained by the enlargement of the driving force to reduce protons in the solution with a negative shift of the conduction band edge of ZAIS NCs and by the quenching of photoexcited ZAIS NCs with an increase in the amount of surface defect sites and/or with the formation of deeper trap sites along with a decrease in the particle size.

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