Subscriber access provided by UNIV OF MISSISSIPPI
Perspective
Graphene Based Membranes for Molecular Separation Liang Huang, Miao Zhang, Chun Li, and Gaoquan Shi J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 24 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 40
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
Graphene Based Membranes for Molecular Separation Liang Huang, Miao Zhang, Chun Li, Gaoquan Shi* Country Collaborative Innovation Center for Nanomaterial Science and Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China
ACS Paragon Plus Environment
1
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 40
ABSTRACT In comparison with traditional chemical separation processes, membrane separation is much simpler and more efficient. An ideal membrane for molecular separation should be as thin as possible to maximize its solvent flux, mechanically robust to prevent it from fracture, and have well-defined pore sizes to guarantee its selectivity. Graphene is an excellent platform for developing size-selective membranes because of its atomic thickness, high mechanical strength, and chemical inertness. In this Perspective, we review the recent advancements on the fabrication of nanoporous graphene membranes and graphene oxide membranes (GOMs) for molecular separation. The methods of fabricating these membranes are summarized, and the mechanisms of molecular separation based on these two types of graphene membranes are compared. The challenges of synthesizing and transferring large-area nanoporous graphene membranes and engineering the performances of GOMs are discussed.
Graphical Abstract
ACS Paragon Plus Environment
2
Page 3 of 40
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
Membrane separation technology plays an important role in water treatment, food processing, chemical and pharmaceutical industries.1–3 So far, the majority of separation membranes are based on polymeric materials, such as cellulose, polyamide and polysulfone.1,4 However, polymeric membranes often show poor tolerance to high temperatures, oxidants, strong acidic/alkaline regents, and/or organic solvents.1,2,4 Therefore, separation membranes with better thermal and chemical stability are highly desirable. Recently, graphene has been confirmed to be an excellent platform for developing size-selective molecular separation membranes because of its atomic thickness, high mechanical strength, and chemical inertness.5–9 Graphene composes of a single layer of carbon atoms arranged in a sp2-bonded aromatic structure.10 The delocalized electron clouds of π-orbitals occupy the voids of aromatic rings in a graphene sheet, thereby effectively preventing the permeation of even the smallest molecule, helium.11,12 Therefore, graphene was tested to be an impermeable material in its pristine state.12 Nevertheless, protons were reported to be able to penetrate through monolayer graphene.13 The impermeability of graphene for molecules makes it is applicable as a barrier layer for gases and liquids14, or to protect metallic surfaces against corrosion.15 For molecular separation, a graphene based membrane has to be functionalized with nanopores or nanochannels through chemical or physical approaches. The typical graphene based materials for this purpose are nanoporous graphene and graphene oxide (GO) membranes (GOMs, Figure 1).16
ACS Paragon Plus Environment
3
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 40
Figure 1. Schematic diagram of the two types of graphene-based membranes. (a) A nanoporous graphene membrane consists of a single layer of graphene with nanopores of defined sizes. Selectivity is achieved by size exclusion or electrostatic repulsion between charged species and the pores. (b) A GOM composed of stacked GO sheets. In a GOM, the transport passages are the interlayer spaces between the sheets, the wrinkles and holes within GO sheets. Reprinted with permission from ref 16. Copyright 2015 RSC Publishing.
Nanoporous graphene membranes with well-defined pores are highly efficient for molecular separation because of their atomic thicknesses Molecular dynamics (MD) simulations have predicted that monolayer graphene with subnanometer pores could act as a highly selective and permeable separation membrane with much higher efficiencies than those of state-of-the-art polymeric filtration membranes.7,17–20 The selective passage of ions through nanoporous graphene is determined by pore sizes, and the electrostatic interaction between the ions and the pores.7,8 Accordingly, ion rejection can be tuned by functionalization of the nanopores (Figure 2a and b). Hydrophilic pores provide higher water flux but lower salt rejection than that of hydrophobic pores (Figure 2c).7 Theoretically, the nanoporous graphene can reject salt ions with a water flux 2 − 3 orders of
ACS Paragon Plus Environment
4
Page 5 of 40
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
magnitude higher than those of commercially available reverse osmosis (RO) membranes (Figure 2c).
Figure 2. Functionalization of the pores in graphene with hydrogen atoms (a) or hydroxyl groups (b). (c) Performance chart for functionalized nanoporous graphene versus existing technologies. Reprinted with permission from ref 7.
In MD simulations, nanopores with a high number density and pre-defined sizes can be ideally introduced into a monolayer graphene membrane. Unfortunately, precisely controlling pore sizes and achieving high pore density on a large-area graphene are technically challenging. Nevertheless, extensive efforts have been devoted to reaching this aim.5,6,21–24 The advantages and limitations of different perforation methods were summarized in Table 1. Focused electron beam irradiation of single layer graphene can create nanopores with controlled sizes (Figure 3a).21 But this approach is inefficient and not scalable for fabricating large-area porous graphene with high pore density.
ACS Paragon Plus Environment
5
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 40
Figure 3. (a) Four adjoining nanopores made by electron beam ablation; the scale bar is 10 nm. Reprinted with permission from ref 21. Copyright 2008 American Institute of Physics. (b) Nanopores created by ion bombardment and subsequent chemical oxidative etching; the scale bar is 1 nm. Reprinted with permission from ref 6. (c) A high concentration of defects exists in CVD graphene (the region between the dashed lines); the scale bar is 1 µm. (d) Magnified view of defective area in CVD graphene (pores can be seen in the graphene lattice, as indicated by red arrows); the scale bar is 10 nm. Panel (c) and (d) reprinted with permission from ref 30. (e, f) Nanopores generated by oxygen plasma etching (e) and focused ion beam (f); the scale bars are 2 and 100 nm. Panel (e) reprinted with permission from ref 22. Copyright 2015 Macmillan Publishers Limited. Panel (f) reprinted with permission from ref 24. Copyright 2014 AAAS.
ACS Paragon Plus Environment
6
Page 7 of 40
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry Letters
Bunch et al. created pores in graphene by ultraviolet-induced oxidative etching and demonstrated the selective gas transport through nanoporous graphene.5 This method is scalable but the pore size distribution is too wide to perform precise molecular sieving, and the number density of pores is also insufficiently high for practical application. O’Hern et al. used ion bombardment to introduce isolated and reactive defects on single layer graphene, and further enlarged these defects to nanopores by controlled chemical oxidative etching.6 The resulting porous graphene possesses high-density nanopores with a size distribution less than 0.2 nm (Figure 3b). The pore size can be tuned by controlling the etching time. The nanopores formed at short etching time can prevent the passing of anions because of the electrostatic repulsion from the negatively charged functional groups at the pore edges. At a longer etching time, the steric size exclusion effect dominated the permeability of the membrane, making salts could pass through, while larger organic molecules were blocked. This work presents the first step towards the production of large-area nanoporous graphene membranes with well-defined pores size distribution. However, in this case, the performances of nanoporous graphene membranes were severely limited by the quality of raw graphene monolayers.
ACS Paragon Plus Environment
7
The Journal of Physical Chemistry Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 40
Table 1. Advantages and limitations of different perforation methods. Method Focused electron beam ablation
Pore size (nm) ~3.5
Advantage
Limitation
Ref.
Tunable and welldefined pore size
Limited to small area
21
Ultraviolet-induced oxidative etching
