Ultrafine monolayer Co-containing layered double hydroxide nanosheets for water oxidation

Abstract

For many two-dimensional (2D) materials, low coordination edges and corner sites offer greatly enhanced catalytic performance compared to basal sites, motivating the search for new synthetic approaches towards ultrathin and ultrafine 2D nanomaterials with high specific surface areas. To date, the synthesis of catalysts that are both ultrathin (monolayer) and ultrafine (lateral size <10 nm) has proven extremely challenging. Herein, using a facile ultrasonic exfoliation procedure, we describe the successful synthesis of ultrafine ZnCo-LDH nanosheets (denoted as ZnCo-UF) with a size ∼3.5 nm and thickness ∼0.5 nm. The single layer ZnCo-UF nanosheets possess an abundance of oxygen vacancies (V_O) and unsaturated coordination sites, thereby affording outstanding electrocatalytic water oxidation performance. DFT calculations confirmed that V_O on the surface of ZnCo-UF enhanced H₂O adsorption via increasing the electropositivity of the nanosheets.

Introduction

In the past few years, two-dimensional (2D) ultrathin nanomaterials (other than graphene) have attracted enormous attention in electrocatalysis [1], [2], photocatalysis [3], [4] and other catalytic applications due to their highly accessible active sites and enhanced catalytic performance compared with conventional 3D nanocatalysts [5]. For example, using ultrasonic exfoliation or solution-phase synthetic routes, monolayer nanosheets of graphitic carbon nitride (g-C₃N₄) [4], hexagonal diatomic boron nitride (h-BN) [6], transition metal dichalcogenides (TMDs) [2], [7] and black phosphorus [8] have been successfully synthesized. In most cases, these 2D materials offered dramatically improved catalytic activity compared with their bulk counterparts. However, the lateral size of most monolayer 2D nanosheets reported to date is typically around several hundred nanometers [9], [10]. Since the active sites for most 2D materials are located on the edge of nanosheets rather than the basal planes [11], the catalytic performance of most 2D materials is still sub-optimal. To maximize catalyst performance, the preparation of ultrathin and ultrafine nanostructured catalysts with lateral size less than 10 nm is the prized target, though highly challenging from a synthetic viewpoint due to high thermodynamic instability of such ultrafine nanosheets.

Layered double hydroxides (LDHs) containing divalent and trivalent transition metal cations such as Fe [12], [13], [14], Co [15], [16], [17], [18], [19], [20] or Ni [12], [21], [22], [23], [24] demonstrate excellent performance for the water oxidation evolution reaction (OER). A NiFe-LDH/CNT nanocomposite was reported to possess a high intrinsic OER activity [25] and a ZnCo-LDH catalyst excellent activity and selectivity in both water and alcohol oxidation [16]. Most reported electrocatalytic studies on LDH nanosheets are for samples with a lateral size more than 30 nm. Control of the ab surface has largely been ignored due to challenges in synthesizing materials with reduced lateral dimensions (<10 nm). Recently, the edges and corners of the LDH nanosheets were identified as the OER active sites [11], prompting research aimed at control of the ab surface size to below 10 nm for enhanced catalytic activity [19], [26]. Very recently, our group reported the first successful synthesis of ultrafine monolayer NiFe-LDH nanosheets [27] (size ∼2.3 nm and thickness ∼0.6 nm) by ultrasonication of monolayer LDH in formamide. Abundant metal and oxygen vacancies imparted the ultrafine nanosheets with semi-metallic character and thus superior charge transfer properties and electrochemical water oxidation performance. However, that synthesis method was based on a monolayer LDH precusor of large lateral dimensions in the ab plane. This work piqued our interest as to whether ultrafine LDH nanosheets could be directly obtained in a single step from multilayered bulk LDH precursors, thereby greatly extending the universality of ultrafine LDH nanosheets for different applications.

Herein, we report the successful synthesis of Co-containing ultrafine monolayer LDH (denoted as ZnCo-UF) from a bulk precursor (the size of ∼700 nm, denoted as ZnCo-Bulk). Ultrasonic exfoliation of ZnCo-Bulk in formamide yielded ZnCo-UF nanosheets with size 3–5 nm and a thickness ∼0.5 nm (Scheme 1). As expected, these ultrafine nanosheets with their abundance of exposed edges exhibited remarkable activities for OER with an overpotential of only 340 mV required to achieve a current density of 5 mA cm⁻¹, much lower overpotential required using the bulk counterpart (530 mV). Oxygen defects and highly exposed active sites imparted ZnCo-UF with high electrical conductivity, further benefitting OER performance.