In the present paper, we have described the interesting behavior of molybdophosphoric acid (H3[PMo12O40], HPMo) in the size-controlled synthesis of silver nanoparticles under UV-irradiation. In this process which is based on the reduction of Ag+ (AgNO3), HPMo plays the role of photocatalyst, reducing agent as well as stabilizer, and propan-2-ol acts as a sacrificial agent. The method allows the rapid synthesis of uniform spherical nanoparticles with an average size that varies between 2.2 and 35.2 nm by altering the silver ion concentration, molar ratio of silver ion to HPMo (or dose of HPMo) and Propan-2-ol amount. It is found that there is a critical ratio for [Ag+]/[HPMo] (i.e. 3.8 in the present case), in which two opposing trends in the size of silver nanoparticles take place.

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