Elsevier

Journal of Membrane Science

Volume 618, 15 January 2021, 118635
Journal of Membrane Science

Functionalized nanoporous graphene membrane with ultrafast and stable nanofiltration

https://doi.org/10.1016/j.memsci.2020.118635Get rights and content

Highlights

  • Water-soluble nanoporous graphene was synthesized and used for membrane fabrication.

  • The membrane can stand harsh conditions under cross-flow filtration.

  • The membrane exhibited ultrafast water permeance (586 Lm-2h-1bar-1) and precise molecular separation (molecular weight cut off: 269 Da).

  • Flux decline of the membrane by filtered molecules can be suppressed in the presence of nanopores.

Abstract

Herein, a functionalized nanoporous graphene (FNG) membrane was developed to mitigate the contemporary issues affecting graphene oxide (GO) membranes, such as the low flux induced by its long tortuosity and poor membrane stability in aqueous solvents. GO was thermally activated at 650 °C to prepare a nanoporous carbon sheet with a turbostratic structure (pore size < 4 nm). Thereafter, the nanoporous graphene (NG) was consecutively functionalized with oxygen-containing groups by KMnO4 treatment and re-dispersed in water to deposit an FNG layer on a porous polymeric support. The FNG membrane exhibited ultrafast water permeance (586 Lm-2h-1bar-1) and precise molecular separation (molecular weight cut off: 269 Da). The membrane performance surpasses the upper bound of previously reported polymers and two dimensional-material-based nanofiltration membranes by the synergistic effect of nanopores and oxygen-containing groups. Furthermore, the practical operation of the FNG membrane is feasible under cross-flow, and water-flux decline by filtered molecules is highly suppressed by the presence of abundant nanopores as compared to conventional GO membranes.

Introduction

Two-dimensional materials, such as graphene oxide (GO), transition metal dichalcogenides, boron nitrides, and MXenes, have been used for nanofiltration membranes due to their precise molecular separation by narrow interlayer spacing [[1], [2], [3], [4]]. GO is particularly promising owing to its excellent mechanical properties, chemical resistance, low biofouling tendencies, and well-established bulk-scale preparation method [[5], [6], [7], [8]]. Despite these advantages, the molecular separation performance, especially solvent permeance, is highly sensitive to the thickness of the stacked GO layer due to the intrinsic barrier property perfectly blocking small molecules, even for hydrogen and helium [9]. Therefore, controlling the structure of the graphene layer is critical to develop nanofiltration membranes with high permeance.

Recently, several approaches have been reported to enhance the permeance of graphene-based nanofiltration membranes. Firstly, various nanomaterials, such as nanostrands, nanoparticles, carbon dots, polymer, graphene oxide nanoribbon (GONR), and diamines, were used to enlarge the interlayer spacing of graphene-based membranes [[10], [11], [12], [13], [14], [15]]. Secondly, nanomaterials with small feature sizes, such as GONR and small GO, were used to fabricate membrane layers to decrease the diffusion length of solvents [15,16]. Although the addition of nanomaterials and small nanosheets effectively widens the nanochannel space, thereby enhancing water permeance, a typical trade-off between permeance and rejection occurs, leading to a high molecular weight cut-off (MWCO) [15]. Graphene membranes were also post-treated via ion beam etching, oxygen plasma etching, and ultraviolet (UV) light to generate nanopores on the basal plane of graphene [[17], [18], [19], [20]]. However, the post-treatment approaches are usually effective for thin graphene layers, such as few-layer graphene synthesized by chemical vapor deposition [[17], [18], [19]], while precise pore generation in large areas remains challenging.

Herein, we develop a facile method to synthesize a nanoporous carbon sheet, which is partially decorated with oxygen-containing groups. The FNG features a turbostratic structure composed of sp3 carbons and abundant nanopores (<4 nm size). The oxygen-containing groups generated by an additional oxidation step improve the dispersion of carbon sheets in water, thereby enabling membrane fabrication by means of environmentally friendly procedures. The nanopores enhance water permeation by decreasing the transport path of water through the stacked graphene layer, as well as act as a molecular sieve to filter molecules larger than their pore sizes. The synergistic effect of functionalization and nanoporous generation on the nanofiltration performance was systematically investigated via both dead-end and cross-flow filtrations.

Section snippets

Preparation of nanoporous graphene

GO powder was prepared by a modified Hummers’ method as used in our previous work [21], and it was loaded in a quartz tube under ambient conditions. The quartz tube was placed in a thermal furnace, which was preset to 650 °C. After heating the sample for 3 min, the quartz tube was removed from the furnace and cooled down to room temperature, and the resulting NG product was collected.

Preparation of functionalized nanoporous graphene

FNG was synthesized from the NG powder via a modified Hummers’ method. NG powder (1 g) was dissolved in sulfuric

Fabrication of functionalized nanoporous graphene membrane

The procedure to fabricate nanofiltration membranes using FNG is illustrated in Fig. 1A. Summarily, the as-prepared GO was placed in a furnace at 650 °C for 3 min. Thermal treatment is effective to activate nanopores on the basal plane of GO by the spontaneous decomposition of oxygen groups at high temperature to leave a defective pore comprising sp3 carbons [22]. Next, the NG powder was functionalized with oxygen-containing groups using a sulfuric acid solution with KMnO4. The oxidation of NG

Conclusions

Thermal activation of GO is an effective method to prepare nanopore carbon sheets, while their poor dispersion in water is one of the main challenges in the fabrication of NG membranes, which require toxic organic solvents. We proved that the post-oxidation method is effective to prepare nanopore carbon sheets dispersible in water even at high concentrations. Therefore, fabrication of high performance nanofiltration membranes is feasible using nanoporous carbon sheets in an eco-friendly manner.

CRediT authorship contribution statement

Junhyeok Kang: Writing - original draft. Yunkyu Choi: Formal analysis. Ji Hoon Kim: Formal analysis. Eunji Choi: Formal analysis. Seung Eun Choi: Formal analysis. Ohchan Kwon: Formal analysis. Dae Woo Kim: Writing - original draft, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2020R1C1C1003289), the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2019R1A6A1A1105566012), the Yonsei University Research Fund of 2019–22-0012, and also in part by Samsung Electronics.

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