Provided are methods of producing cobalt-based nanoparticles (CoxBy NPs) of a pre-selected diameter. The methods include reducing Co2+ ions with a sodium borohydride (NaBH4) solution having a selected ratio of tetrahydroxyborate (B(OH)4−；) to tetrahydroborate (BH4−；) based on the pre-selected diameter, where the ratio of B(OH)4−； to BH4−； is positively correlated with the pre-selected diameter. Also provided are methods of using the CoxBy NPs to produce hollow metal nanospheres (HMNs). Methods of producing CoxBy NP core/metal shell structures are also provided, such methods including combining in an anaerobic galvanic exchange reaction a deaerated solution including CoxBy NP scaffolds and a deaerated solution including a metal. Also provided are methods of producing HMNs from the CoxBy NP core/metal shell structures. Compositions and kits that find use in practicing the methods of the present disclosure and using HMNs produced in accordance with the methods of the present disclosure, are also provided.
