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: alpha and beta. The single-layer alpha-Co(OH)(2) nanosheet has been prepared by exfoliating directly alpha phase layered Co(OH)(2). However, due to theoretical barriers, a single-layer beta-Co(OH)(2) nanosheet has not been achieved so far. In this article, phase transformation during exfoliation of layered Co(OH)(2) from alpha to beta is observed and a single-layer beta-Co(OH)(2) nanosheet with a thickness of similar to 1.1 nm is prepared through phase transition of layered alpha-Co(OH)(2) nanocones in a mild wet chemical process for the first time, with a nearly 100% yield. The as-prepared single-layer beta-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/beta-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 beta-Co(OH)(2) nanosheets at the current density of 1 A/g. The electrode also demonstrates the excellent rate performance and long cycle life.