Nanostructures DOI: 10.1002/anie.200803202 Shape-Controlled Synthesis of Gold Nanoparticles in Deep Eutectic Solvents for Studies of Structure–Functionality Relationships in Electrocatalysis** Hong-Gang Liao, Yan-Xia Jiang,* Zhi-You Zhou, Sheng-Pei Chen, and Shi-Gang Sun* Gold-based catalysts have attracted intense interest in recent years following the discovery [1] that small supported Au nanoparticles (NPs) can be effective catalysts for CO oxidation at low temperatures. Gold-based catalysts have been widely used and are regarded as a new generation of catalysts. [2] ThepropertiesofAuNPsdependstronglyontheir size and shape, which determine the surface structure of the particles.Fundamentalstudiesofsingle-crystalplanesofface- centered-cubic (fcc) metals demonstrated that the high-index planespossessahighdensityoflowcoordinationnumberstep atoms in comparison with low-index planes such as {111}, {100}, and even {110}, and therefore exhibit high activity for breakingchemicalbonds. [3] Tianetal. [4] reportedrecentlythat tetrahexahedral-shaped Pt NPs display high catalytic activity and stability, as the tetrahexahedral Pt nanoparticle is bounded by a {730} surface and other high-index facets. Although the shape-controlled synthesis of Au NPs has been extensively studied, [5] the procedures often involve surfac- tants and Au seeds in solutions. Moreover, the shapes of the synthesized Au NPs are mainly cubes, tetrahedra, octahedra, and rhombic dodecahedra that are enclosed by low-index facets.SeveralstarlikeAuNPsboundedwithlow-index(111) facets have also been reported recently, but their shapes are not uniform and perfect. [6,7] It is known from crystal growth law that the growth rate along the direction perpendicular to the high-index planes is usually much faster than that to the low-index planes, so the high-index planes will be eliminated gradually during the crystal growth. Therefore, it is always challenging to develop shape-controlled syntheses of NPs bounded with high-index facets. Deep eutectic solvents (DESs) [8] were first reported by Abbott and co-workers. They found that substituted quater- naryammoniumsaltsmixedwithhydrogen-bonddonorssuch as amides can form liquids at ambient temperatures. These liquids have properties similar to those of ionic liquids, namely,highconductivity,viscosity,surfacetensions,polarity, and thermal stability and negligible vapor pressure. Unlike theionicliquids,DESscanneverthelessbeeasilypreparedat low cost and with high purity. Some of the hydrogen-bond donors are common bulk commodity chemicals such as urea and oxalic acid, which are suitable for large-scale processes. DESs form extended hydrogen-bond systems in the liquid state and are therefore highly structured “supramolecular” solvents.Thisspecialqualitycanbeusedtoformwell-defined, ordered nanoscale structures. Owing to the high thermal stability and low vapor pressure of DESs, reactions can be conducted at temperatures well beyond 100 8C in nonpres- surized vessels. Low interface energies for particles can be translated into good stabilization. DESs are promising solvents to be used in shape-controlled synthesis of nano- particles.However,thereareasyetveryfewreportsoftheuse of DESs in the synthesis of nanomaterials. [9] Herein we report a novel route of shape-controlled synthesis of gold NPs without the use of any surfactants or seedsbutwithaDESassolvent.Star-shapedAuNPsthatare bounded with (331) and vicinal high-index facets were successfully synthesized for the first time. The monodisperse star-shapedgoldNPswereobtaineddirectlybythereduction of HAuCl 4 by l-ascorbic acid at room temperature in the DES.AuNPsofvariousshapeandsurfacestructureincluding snowflake-like NPs and nanothorns can be obtained simply by adjusting the content of water in the DES. Furthermore, the electrocatalytic properties of the synthesized Au NPs were tested by using the electroreduction of H 2 O 2 asaprobe reaction,andithasdemonstratedthatthestar-shapedAuNPs exhibited a much higher catalytic activity than other shaped NPs and polycrystalline Au. The as-prepared samples were examined using scanning electron microscopy (SEM, LEO-1530). It was found that over 40% of the synthesized Au NPs exhibited regular pentagonal symmetry and displayed a striking beauty of the star-shaped particle, as illustrated in the Figure 1. The size of these Au NPs was about 300nm. High-magnified pictures at different viewing angles (FigureS2 in the Supporting Infor- mation) illustrate that the star-shaped Au NPs are flat and haveaslightlythickercenter(about90nm).Besidesthewell- defined pentagonal star shape, other star-shaped Au NPs of three,four,ormultiplebranchesarealsoobservedinFigure1, which may be considered as a deformation of the pentagonal star shape during crystal growth. Energy-dispersive X-ray spectroscopy (EDS) indicates that the NPs are composed of onlygold(seeFigureS1aintheSupportingInformation).We have also traced the synthetic process by using UV/Vis [*] Dr.H.G.Liao,Prof.Y.X.Jiang,Z.Y.Zhou,S.P.Chen,Prof.S.G.Sun State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University Xiamen 361005 (China) Fax: (+ 86)592-2180181 E-mail: yxjiang@xmu.edu.cn sgsun@xum.edu.cn [**] This work was supported by the National Natural Science Foundation of China (Grant Nos. 20433040, 20573085, and 20673091) and the “973” Program (No. 2009CB220102). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200803202. Communications 9100 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2008, 47, 9100 –9103