Stage-1 cationic C₆₀ intercalated graphene oxide films

Highlights

• A stage-1 intercalation of cationic C₆₀ into graphene oxide films is reported.

• Intercalation driven by ion exchange expands the interlayer distance of the films.

• Intercalated films remain stable in water and various organic solvents.

• Water vapor transports through the intercalated films almost freely.

• Heating the film at 2700 °C converted it into highly graphitized structures.

Abstract

A “stage-1” intercalated film has been made by the ion exchange of pyrrolidinium-functionalized C₆₀ (C₆₀(Py)ⁿ⁺) into centimeter-wide, micrometer-thick air-dried graphene oxide (G-O) films composed of tens of thousands of layers of stacked/overlapping G-O platelets, denoted [C₆₀(Py)ⁿ⁺]G-O films. Spontaneous intercalation by ion exchange of one layer of C₆₀(Py)ⁿ⁺ between adjacent G-O layers expands the interlayer spacing of the films from 0.74 nm to 1.46 nm. The films remain intact in water and various organic solvents, which is likely due to a strong affinity between C₆₀(Py)ⁿ⁺ and G-O. Membranes made of the films showed a 6.8 times faster water vapor permeation rate (allowing the vapor to transport through the membrane almost freely) and a 10.5 times faster liquid water permeation rate than G-O membranes. Heating the films at 2000 °C under applied pressure or at 2700 °C without physical confinement converted them into highly graphitized structures.

Introduction

“Bottom up” assembly of carbon “building blocks” can lead to new structures whose properties can then be explored. Examples of such materials include the “peapod” (an array of C₆₀ encapsulated in carbon nanotubes) [1] and the “buckyball sandwich” (one layer of C₆₀ confined between two layers of graphene) [2]. In addition, a graphite intercalation compound (GIC) with C₆₀ intercalated between adjacent layers has been predicted to be stable [3], although it could not be formed by directly exposing graphite to C₆₀ vapor or a solution of C₆₀ [4]. In attempts to intercalate C₆₀, well-known GICs were first formed, followed by their exposure to a C₆₀ solution for the possible formation of co-intercalated compounds [[4], [5], [6]]. Achieving the assembly of different forms of carbon into new long-range ordered structures on a macroscopic scale is important.

Graphene oxide (G-O) is an oxidized form of graphene, typically prepared by the oxidation of graphite flakes to graphite oxide and subsequent exfoliation into single layers [7]. Solid ‘G-O films’ (or ‘papers’) composed of stacked and overlapping layers of G-O platelets have a lamellar structure that can also be densely packed, and has an interlayer spacing that depends on the amount of intercalated water [8]. G-O films (or graphite oxide particles) can act as a “host” to accommodate intercalants because the interlayer galleries facilitate “guest” species entering and binding to the G-O layers. Oxygen-containing groups on G-O give it a negative charge that is balanced by hydronium ions, yielding an electrical double layer at the water/G-O interface [9]. Ion exchange allows the selective intercalation of organic or inorganic cationic species into G-O films, which causes an increase in interlayer spacing and film thickness depending on the size and orientation(s) of the intercalants [[10], [11], [12], [13], [14], [15], [16]].

Membranes produced from G-O films with modifiable interlayer spacing and stability in a range of solvents are useful in a variety of applications including water treatment [17,18], gas separation [19,20], and organic solvent nanofiltration [21]. As reported, the separation of molecules and ions can be by size and/or charge exclusion [22], where key factors to evaluate the performance of these membranes are the permeate flux, selectivity, and stability [23]. The stability is a major issue facing their application as G-O films tend to experience significant swelling in polar solvents [24]. In most cases, the swelling of the interlayer spacing will sacrifice the rejection performance for species that are smaller or similar in size [18]. One way to improve stability is through G-O sheets being cross-linked or intercalated to create a range of stable membranes with different interlayer spacing and permeance [23,25,26].

Here, we report the preparation of pyrrolidinium-functionalized C₆₀ (C₆₀(Py)ⁿ⁺) intercalated G-O films by ion exchange after immersing G-O films into an aqueous C₆₀(Py)ⁿ⁺ salt solution, Fig. 1a (inset). To the best of our knowledge, this is the first work in which C₆₀ derivatives are directly intercalated into large-area films composed of stacked/overlapped sheets of any type. An air-dried intercalated film ([C₆₀(Py)ⁿ⁺]G-O film) shows an increased interlayer spacing and a “stage-1”-like intercalation of C₆₀(Py)ⁿ⁺ between adjacent G-O layers. The structure, chemical composition, and thermal and chemical stabilities of the film are presented and discussed below. Water permeation through the G-O membranes and the [C₆₀(Py)ⁿ⁺]G-O membranes showed a significantly higher flux for the latter. The thermal decomposition and graphitization of the film have also been studied as described below.