Phase transformation guided single-layer β-Co(OH)₂ nanosheets for pseudocapacitive electrodes.

It is known that Co(OH)2 can be crystallized into a layered structure with two polymorphs: α and β. The single-layer α-Co(OH)2 nanosheet has been prepared by exfoliating directly α phase layered Co(OH)2. However, due to theoretical barriers, a single-layer β-Co(OH)2 nanosheet has not been achieved so far. In this article, phase transformation during exfoliation of layered Co(OH)2 from α to β is observed and a single-layer β-Co(OH)2 nanosheet with a thickness of ∼1.1 nm is prepared through phase transition of layered α-Co(OH)2 nanocones in a mild wet chemical process for the first time, with a nearly 100% yield. The as-prepared single-layer β-Co(OH)2 nanosheets are assembled with graphene oxide to form an all-two-dimensional materials-based composite for use as an electrode for the pseudocapacitor. The reduced graphene oxide/β-Co(OH)2 composite exhibits a high specific capacitance up to 2080 F/g scaled to the total mass of the electrode or 3355 F/g scaled to the active mass of β-Co(OH)2 nanosheets at the current density of 1 A/g. The electrode also demonstrates the excellent rate performance and long cycle life.