We show that reaction of aqueous cadmium cations with phosphotungstate Keggin anions leads to the growth of highly dense and crystalline cadmium phosphotungstate colloidal particles with needlelike morphology of submicron dimensions. Further treatment of the crystalline cadmium phosphotungstate colloidal particles with H2S gas leads to the in-situ growth of US nanoparticles of 5-6 nm sizes without disruption to the host Keggin ion colloidal particle morphology. Alkali treatment results in dissolution of the PTA template, leaving behind fused US nanoparticles in the form of nanorods. The metal cation directed morphology variation of Keggin ion colloidal particles and their use as a new class of inorganic scaffolds for the synthesis of nanomaterials are salient features of the work with implications in crystal engineering and nanocomposites design for different applications.