The design and fabrication of efficient Pt-based electrocatalysts with ultralow-loading but high-mass-activity for hydrogen evolution reaction (HER) in alkaline media is of significant importance. Herein, we report the monodispersed platinum nanoparticles (PtNPs) with an ultralow Pt loading (3.6 mu g per electrode area (cm(2))) embed in Ni3S2-containing hollow carbon spheres as HER catalysts. Notably, the as-prepared electrocatalyst exhibits a mass activity of 7.6 A g(Pt)(-1) at an overpotential of 70 mV, which is 8.5 times higher than that of commercial Pt/C and is among the highest levels of reported Pt-based HER electrocatalysts in alkaline conditions. This electrocatalyst also exhibits excellent HER performance with an overpotential of 42 mV reaching an area current of 10 mA cm(-2) and significantly improved stability. The outstanding HER performance is attributed to the hollow and porous carbon spherical structure, as well as highly dispersed PtNPs that provide more active sites and facilitate proton/electron transport. Meanwhile, the nickel-containing layered precursors convert to some Ni3S2 NPs which guarantees the formation of Pt-Ni3S2 heterostructures on the carbon substrate and thus further enhance the activity of PtNPs. This work provides an effective strategy to simultaneously regulate the nanostructure and active site of Pt-containing carbon-based HER electrocatalysts. (C) 2019 Elsevier Ltd. All rights reserved.