title: Metallic single-unit-cell orthorhombic cobalt diselenide atomic layers: robust water-electrolysis catalysts.
abstract: The bottleneck in water electrolysis lies in the kinetically sluggish oxygen evolution reaction (OER). Herein, conceptually new metallic non-metal atomic layers are proposed to overcome this drawback. Metallic single-unit-cell CoSe2 sheets with an orthorhombic phase are synthesized by thermally exfoliating a lamellar CoSe2 -DETA hybrid. The metallic character of orthorhombic CoSe2 atomic layers, verified by DFT calculations and temperature-dependent resistivities, allows fast oxygen evolution kinetics with a lowered overpotential of 0.27 V. The single-unit-cell thickness means 66.7 % of the Co(2+) ions are exposed on the surface and serve as the catalytically active sites. The lowered Co(2+) coordination number down to 1.3 and 2.6, gives a lower Tafel slope of 64 mV dec(-1) and higher turnover frequency of 745 h(-1) . Thus, the single-unit-cell CoSe2 sheets have around 2 and 4.5 times higher catalytic activity compared with the lamellar CoSe2 -DETA hybrid and bulk CoSe2 .
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China).
authors.forename: Liang
authors.initials: L
authors.lastname: Liang
authors.affiliation: National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029 (P.R. China).
authors.forename: Hao
authors.initials: H
authors.lastname: Cheng
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China).
authors.forename: Fengcai
authors.initials: F
authors.lastname: Lei
authors.affiliation: Paul Scherrer Institut, 5232 Villigen PSI (Switzerland).
authors.forename: Jun
authors.initials: J
authors.lastname: Han
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China).
authors.forename: Shan
authors.initials: S
authors.lastname: Gao
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China).
authors.forename: Chengming
authors.initials: C
authors.lastname: Wang
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China). yfsun@ustc.edu.cn.
authors.forename: Yongfu
authors.initials: Y
authors.lastname: Sun
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China).
authors.forename: Shaista
authors.initials: S
authors.lastname: Qamar
authors.affiliation: National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029 (P.R. China). sqwei@ustc.edu.cn.
authors.forename: Shiqiang
authors.initials: S
authors.lastname: Wei
authors.affiliation: Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230026 (P.R. China). yxie@ustc.edu.cn.
authors.forename: Yi
authors.initials: Y
authors.lastname: Xie
keywords: atomic layers; cobalt diselenide; electrocatalysis; water splitting