Enhanced electrocatalytic activity for hydrogen evolution reaction from self-assembled monodispersed molybdenum sulfide nanoparticles on an Au electrode† Tanyuan Wang, a Lu Liu, a Zhiwei Zhu, a Pagona Papakonstantinou, b Jingbo Hu, c Hongyun Liu c and Meixian Li * a Ultrasmall molybdenum sulfide nanoparticles with diameters of 1.47  0.16 nm were fabricated from bulk MoS 2 by a combination of ultrasonication and centrifugation. The nanoparticles were then assembled on an Au surface to form a film with high electrocatalytic activity for hydrogen evolution reaction (HER). A Tafel slope of 69 mV per decade was measured for this film and the onset potential was estimated to be 0.09 V. The small loading (1.03 mg cm 2 ) and the high current density (0.92 mA cm 2 at h ¼ 0.15 V) demonstrated extremely high catalytic efficiency. X-ray photoelectron spectroscopic results revealed that the assembled nanoparticle film was sulfur enriched with abundant S edges and a structural rearrangement of the S rich particles might occur during the self-assembly process, resulting in significantly enhanced electrocatalytic activity for HER. Electrochemical impedance measurements suggested that the assembling process optimized the conductivity of the nanoparticle film, which contributed to the enhanced HER catalytic activity. Our research has provided a new way to synthesize active molybdenum sulfide nanoparticles for HER and a new approach to achieve enrichment of S edges on molybdenum sulfide, which might have potential use not only for electrocatalytic HER, but also for photoelectrocatalytic HER and plasmon-enhanced water splitting. Broader context Hydrogen produced by the water splitting process could potentially address the needs for the sustainable production of fuels in a manner that is renewable and carbon-free. However, cheap catalysts are required to overcome the large overpotential for hydrogen evolution reaction (HER) in an affordable manner. Recent studies have shown that sulfur edges of MoS 2 are quite active for HER. As a result, much effort has been focused on trying to acquire MoS 2 or MoS x nanomaterials with a plethora of sulfur edges. Here, we demonstrate a novel approach for synthesizing monodispersed molybdenum sulde nanoparticles with ultrasmall diameters and a new strategy for achieving enrichment of active S edges on molybdenum suldes by exploiting the covalent bonding of the nanocatalysts with the underlying gold electrode. The work is benecial not only for the design of highly efficient molybdenum sulde based HER catalysts but also for the synthesis of new catalysts for photoelectrocatalytic HER and plasmon-enhanced water splitting. 1 Introduction The rapid depletion of fossil fuels and growing environmental concerns have created an enormous worldwide demand for alternative clean energy technologies. Compared with tradi- tional fossil fuels, hydrogen is considered as an ideal energy carrier since it is clean and renewable. 1,2 The most promising way to create hydrogen is through the splitting of water by either light or electricity. 3,4 However, catalysts are necessary to achieve a highly efficient hydrogen production due to the existence of a large overpotential for hydrogen evolution reaction (HER). Pt- group metals are the most efficient catalysts for HER, but they are rare and expensive. In order to maintain the development of hydrogen energy, it is of great importance to nd cheap cata- lysts with high HER activity. Molybdenum disulde (MoS 2 ), a low cost and easily obtained material with good catalytic prop- erty for HER, has attracted great attention. 5–10 MoS 2 is well known as a solid lubricant and also as a good catalyst for hydrodesulfurization reaction. 11 Recent studies have shown that its sulfur edge is quite active for HER. 12–14 As a result, much effort has been focused on trying to acquire MoS 2 or MoS x with high catalytic activity for HER. 5,6,15 The most common pathways to achieve this were through increasing the edges of MoS 2 or MoS x by introducing a nanosize dimension 16,17 or an amorphous structure 18 and through optimizing the conductivity a College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R.China. E-mail: lmwx@pku.edu.cn b School of Engineering, Engineering Research Institute, University of Ulster, Newtownabbey BT37 0QB, UK c Department of Chemistry, Beijing Normal University, Beijing 100875, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ee23513g Cite this: Energy Environ. Sci., 2013, 6, 625 Received 9th July 2012 Accepted 21st November 2012 DOI: 10.1039/c2ee23513g www.rsc.org/ees This journal is ª The Royal Society of Chemistry 2013 Energy Environ. Sci., 2013, 6, 625–633 | 625 Energy & Environmental Science PAPER