96 | Mater. Chem. Front., 2018, 2, 96--101 This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2018 Cite this: Mater. Chem. Front., 2018, 2, 96 Crystalline–amorphous Co@CoO core–shell heterostructures for efficient electro-oxidation of hydrazine† Xiaodong Yan, ‡ a Yuan Liu,‡ b Jinle Lan, c Yunhua Yu, c James Murowchick, d Xiaoping Yang* c and Zhonghua Peng * a Metal–metal oxide core–shell heterostructures have exhibited outstanding performances in many realms, especially in catalysis, due to enriched phase interfaces and strong metal/metal oxide interactions. Herein, Co@CoO core–shell heterostructures with a crystalline Co core and an amorphous CoO shell have been prepared using a simple solution reduction process and shown to be effective catalysts for the electro-oxidation of hydrazine in alkaline media. The synergistic effect between the crystalline metallic Co core and the amorphous CoO shell leads to a small onset potential of À1.10 V vs. Ag/AgCl. In addition, the Co@CoO electrode has been shown to exhibit good long-term catalytic stability. 1. Introduction Surface properties of a solid are contingent on the local structures near the surface regions that play a critical role in surface reactions, 1–3 such as electro-catalysis, photo-catalysis and surface energy storage. Tuning the structure of the existing materials can thus potentially enhance their inherent properties, and may even add new functional properties to them. 3–21 For example, it has been shown that deliberately introducing an amorphous surface layer on TiO 2 nanocrystals extends its optical absorption from ultraviolet to visible spectrum with enhanced photocatalytic activity 6 and can also lead to strong microwave absorption. 7,8 Partially amorphized MnMoO 4 by hydrogenation treatment shows markedly higher catalytic activity towards the hydrogen evolution reaction and a 17-fold increase in specific capacitance as compared to MnMoO 4 crystals. 9 Incorporation of a small number of heteroatoms (e.g. Fe, Co, and Ni) into the framework of metal sulfides can promote the catalytic activity for the hydrogen evolution reaction by reducing the kinetic energy barrier of H atom adsorption. 12,13 Electrochemical tuning enables olivine-type lithium transition-metal phosphates to have high catalytic activity towards water electrolysis due to the increase in active site density, valency of the transition metal centers, and covalent hybridization between the M-3d and O-2p states. 14 Multi-shelled hollow structures have recently been attracting much attention owing to their unique architectures and highly tunable properties. 15–19 For instance, the capacitance of multi- shelled manganese oxide (Mn 2 O 3 ) hollow microspheres is tunable by manipulating the shell number. 18 Another strategy to promote surface reactions is to control the phase composition in the bulk or introduce phase interfaces near the surface regions where surface reactions occur. 22–38 For instance, compared to single-phase catalysts, both metallic and non-metallic alloys are believed to have higher intrinsic activity towards hydrogen evolution 22–24 and electro-oxidation of hydrazine. 25–27 Improved photocatalytic activity is also observed in biphasic TiO 2 nanocrystals owing to the enhanced charge separation driven by the difference in band gaps of different TiO 2 phases. 28,29 Recently, introducing phase interfaces near the surface regions of solid materials has been of great interest due to the unique physicochemical properties of the phase interface. 31–39 Among them, core–shell heterostructures have attracted more and more attention. 20,33–38,40 For example, Ru@Pt core–shell nano- particles show an unprecedentedly low light-off temperature (351) for preferential oxidation of carbon monoxide in hydrogen. 33 Ni@NiO and Co@Co 3 O 4 core–shell heterostructures present out- standing catalytic activity towards hydrogen evolution, 34–36 as the metal/oxide interface can promote water dissociation and hydrogen adsorption. 30 Similarly, Au@Co 3 O 4 and NiFe@NiFeO x core–shell nanocrystals demonstrate enhanced catalytic activity towards the oxygen evolution reaction owing to the synergistic effect provided by the core–shell structure. 37,38 a Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri 64110, USA. E-mail: PengZ@umkc.edu b State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China c State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: yangxp@mail.buct.edu.cn d Department of Geosciences, University of Missouri – Kansas City, Kansas City, Missouri 64110, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00401j ‡ These authors contributed equally. Received 1st September 2017, Accepted 27th October 2017 DOI: 10.1039/c7qm00401j rsc.li/frontiers-materials MATERIALS CHEMISTRY FRONTIERS RESEARCH ARTICLE