DOI: 10.1002/chem.201301406 Size-Dependent Enhancement of Electrocatalytic Oxygen-Reduction and Hydrogen-Evolution Performance of MoS 2 Particles Tanyuan Wang, [a] Dongliang Gao, [a] Junqiao Zhuo, [a] Zhiwei Zhu, [a] Pagona Papakonstantinou, [b] Yan Li, [a] and Meixian Li* [a] Introduction The energy demands in our society have increased greatly during the recent years and will continue to increase due to improvements in economic growth and our living standards. Therefore, energy conversion and storage technologies that would possibly offer us clean and sustainable energy to meet our energy demands have been one of the most attrac- tive research fields. [1] In this respect, the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR) are two important reactions in the field of energy conversion. They are of key significance in the transformation of solar energy or electricity to chemical energy and the conversion of the chemical energy to electricity. [2] For example, they are central to the operation of direct solar and electrolytic water-splitting devices as well as to fuel cells and air batter- ies. [3] Both ORR and HER reactions are sluggish in nature and traditionally require the use of precious metal catalysts such as platinum. [4] The high cost and scarcity of the precious metals greatly hinder the large-scale implementation of these materials. Thus it is of great importance to develop ORR and HER catalysts based on non-noble metals. A great many materials have been tested for the ORR or HER, including various transition-metal complexes, [5] metal oxides, [6] carbon nanomaterials doped with nonmetallic ele- ments [7] or polymers. [8] Although these materials have shown interesting variations in catalytic behaviour for either ORR or HER, none of them has shown high catalytic activity for both reactions as is the case for precious metals. If a materi- al is both ORR and HER active, it would not only turn clean hydrogen energy into electricity efficiently, but would also play a key role in the production of hydrogen, and therefore it would be of central importance to the acquisi- tion of clean and sustainable energy. MoS 2 is a kind of layered material that is similar to gra- phene. Earlier studies have indicated that its edges are pre- ferred sites for the chemisorption of O 2 . [9] More significantly, recent work has proven that it could be active for HER even though the bulk MoS 2 is inert. [10] All of the above imply that it might be a potential catalyst for both ORR and HER. Here we demonstrate an extremely easy and con- venient way to prepare different sizes of MoS 2 particles from inert and easily available bulk MoS 2 by a combination of sonication and centrifugation. The influence of the size effect on ORR and HER has been investigated systemati- cally. It was found that the relatively small MoS 2 nanoparti- cles show a four-electron reaction route for ORR and a low overpotential for HER, which could have potential applica- tions in energy conversion and storage fields. Abstract: MoS 2 particles with different size distributions were prepared by simple ultrasonication of bulk MoS 2 followed by gradient centrifugation. Relative to the inert microscale MoS 2 , nanoscale MoS 2 showed significantly improved catalytic activity toward the oxygen-reduction reaction (ORR) and hydrogen-evolution reaction (HER). The decrease in particle size was ac- companied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four-electron ORR process and an HER onset po- tential of 0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size-depend- ent effect of the ORR activity of MoS 2 , and a four-electron transfer route was found. The exposed abundant Mo edges of the MoS 2 nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS 2 catalysts that could easily be scaled up. Hence, it opens up possi- bilities for wide applications of MoS 2 nanoparticles in the fields of energy conversion and storage. Keywords: electrocatalysis · hydro- gen evolution · molybdenum · nanoparticles · reduction [a] T. Wang, D. Gao, J. Zhuo, Prof. Z. Zhu, Prof. Y. Li, Prof. M. Li College of Chemistry and Molecular Engineering Peking University, Beijing 100871 (P.R. China) E-mail : lmwx@pku.edu.cn [b] Prof. P. Papakonstantinou School of Engineering, Engineering Research Institute University of Ulster, Newtownabbey, BT37 0QB (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301406. Chem. Eur. J. 2013, 19, 11939 – 11948 # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 11939 FULL PAPER