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Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms' 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance. S ecuring renewable and reliable sources of clean energy is one of the world's foremost challenges. Addressing this challenge is not only critical for the global economy but will also aid in the mitigation of environmental and health hazards caused by fossil fuels 1 . Hydrogen is the cleanest fuel available and is believed to be one of the most promising energy sources of the twenty-first century 2,3 . However, the majority of the hydrogen produced today is derived from steam-reformed methane, which is sourced from fossil reserves and produces a substantial amount of CO 2 (ref. 4). The production of hydrogen from water electrolysis is a promising alternative to the current CO 2 -emitting fossil fuel-based energy systems 5,6 .Platinum (Pt)-based catalysts are generally considered to be the most effective electrocatalysts for the hydrogen evolution reaction (HER) 5,7 . Unfortunately, Pt is expensive and scarce, limiting the commercial potential for such catalysts. The development of active, stable and inexpensive electrocatalysts for water splitting is a key step in the realization of a hydrogen economy, which is based on the use of molecular hydrogen for energy storage.Significant effort has been devoted to the search of non-preciousmetal-based HER catalysts, including sulfide-based materials 8-11 , and C 3 N 4 (refs 12-14). Although these candidate materials show promising activities for the HER, the activities of these catalysts in their present form are insufficient for industrial applications 15 .To overcome the challenges associated with the Pt HER catalysts and to drive the cost of H 2 production from water electrolysis down, it is very important to markedly decrease the Pt loading and increase the Pt utilization efficiency. Currently, supported Pt nanoparticles (NPs) are typically used to promote Pt activity towards the HER. Unfortunately, the geometry of the NPs limit the majority of the Pt atoms to the particle core, deeming them ineffect...

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.