Perforation routes towards practical nano-porous graphene and analogous materials engineering

Abstract

Nano-perforated graphene sheets have emerged as exciting two-dimensional materials for a broad range of scientific and commercial purposes, due to their modified physicochemical properties as compared to native graphene materials. Nanoporous graphene sheets as a class of two-dimensional materials with thicknesses ranging from sub-nanometre to few tens of nanometres, possess high specific surface areas and porous mesh structures with tuneable porosity levels. These properties lead to high densities of unsaturated carbon edges around the pores, making them attractive candidates for applications such as energy storage, separation, sensing or catalysis. Several perforation methodologies have been reported to sculpt pores across graphene structures via etching or guided growth mechanisms. This review focuses on current and emerging nano-perforation methodologies for the two-dimensional graphene materials, and discusses controllable porosity parameters in terms of physical pore size and surface pore density across 2D materials. The relationship between perforation methodology and the achieved porosity level is also discussed and related to electronic or surface reactivity properties. Suggestions towards perforation methodologies in relation to targeted pore size and density, as well as the current challenges hindering scalability of engineering the nanoporous graphene and other similar two-dimensional materials are also highlighted.

Introduction

The nano-perforation of two-dimensional (2D) materials offers opportunities to develop fine nano-architectures with controlled porosities, specific pore dimensions and low tortuous pathway [1,2]. Nano-porosity may be generated as a tool for application in separation or energy storage, towards confinement, diffusion, capture or catalysis by careful alteration of the properties of native materials [3,4].

Graphene nanomaterials are primarily synthesized by epitaxial chemical vapour deposition (CVD) [5], mechanical cleavage [6], liquid exfoliation [7] or chemical exfoliation [8]. Graphene nanosheets may be formed in a wide range of shapes and lateral sizes, stacking from sub-nanometre levels [6] to tens of nanometres in thickness [9]. The surface of native graphene sheets is typically seeded with intrinsic defects, generated at the boundaries between sheets or grains, representing unsaturated sites induced during mechanical exfoliation or chemical etching [10]. Partial and localized amorphization across graphene sheet structures offer favourable sites to graft functional groups, which can be oxygen or nitrogen-rich, leading to altered physicochemical and electrical properties compared to the pristine material [11]. In addition, although pristine graphene does not exhibit any valence or conduction bands, and therefore cannot support an electronic band-gap, the formation of such defects or pores, as well as the combination of suited substrates such as metallic, semiconducting materials, were recently found to support the formation of very large band gap materials [12,13].

However, imperfections present within graphene nanostructures are normally randomly distributed across the graphitic basal plane and may be chemically or physically unstable [14]. The defective sites may be utilized to engineer well-ordered porosity at the nanoscale leading to the formation of nanoporous graphene sheets. The porosity level of nanoporous graphene material plays an essential role in the development of tuneable energy-gap [15], specific surface area [16], and chemical reactivity [17], which may be controlled by the dimension of the pores and their distribution across the graphene nanostructure.

Nanoporous graphene materials have been utilized in applications such as separation [[18], [19], [20]], sensing [[21], [22], [23]], and energy generation or harvesting and storage [[24], [25], [26]]. A key variable in these applications resides in the variety of nanoscale morphologies, specific surface areas and chemistries different from the native graphitic structures. Engineering pores within the sub-nanometre size range was found to be effective for water treatment [20] or molecular selectivity in the gas separation [23]. Not only the physical dimension of pores, but also the thickness of the perforated graphene sheets can provide short diffusion pathways for and thus, higher transport rate for ions in lithium-ion battery [25], or electrolyte for supercapacitor [26] applications.

Fabrication of nanoporous graphene materials from pristine graphene or native graphitic materials can be achieved by various perforation mechanisms, such as etching, wherein several adjacent carbon atoms are removed, or by guiding the nucleation process of graphitic structures, i.e. nucleating carbon clusters around these locations to define vacancies in former locations [46]. During etching, the excitation energy must overcome the energy activation threshold of the chemical bonds, which is the minimum energy to ablate atoms from the graphitic lattice. This energy may be delivered via irradiation or kinetic [4,18,[27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]], thermal [24,[39], [40], [41], [42]] or chemical sources [26,[43], [44], [45], [46], [47], [48], [49], [50]]. The guiding mechanism is also performed using a porous template during the growth to define the sites of the nanopores location, supporting the engineering of pores in a controlled manner [51]. Such mechanisms are, however, restricted to the catalytic activity of the reactive substrates used for pore formation [52].

Previous reviews in this area have provided a summary of various synthesis routes to make nanoporous graphene, with few representative applications [[53], [54], [55], [56], [57]] along with the accuracy of perforation methodologies in controlling the pore formation [54]. Furthermore, the exceeding potential of nanoporous graphene membranes for bio-separation applications, over other inorganic membranes [57] was highlighted. Importantly however, pore size distributions, surface pore densities, and configurations of the engineered nanoporous graphene materials, have not been correlated to the perforation methodology yet [53].

This review provides a systematic and critical analysis of various current and emerging 2D nano-perforation methodologies. The range of nanoporous graphene generated are presented in terms of mean pore size distributions, in micro, nano, meso and macro ranges [58,59]. The current research challenges to yield scalable solutions towards large-scale and reproducible perforation with a high-precision control of the final pore structure and stability are further discussed. Challenges in both formation and characterization of pores and their associated properties in these materials are also discussed considering the reproducibility of the perforation technique and the target pore size distributions and densities. Finally, upcoming research directions of perforating 2D materials other than graphene are highlighted, including the variations in their physicochemical properties.