Selective Separation of Metal Ions via Monolayer Nanoporous

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Selective Separation of Metal Ions via Monolayer Nanoporous Graphene with Carboxyl Groups Zhan Li, Yanqi Liu, Yang Zhao, Xin Zhang, Lijuan Qian, Longlong Tian, Jing Bai, Wei Qi, Hui-Jun Yao, Bin Gao, Jie Liu, Wangsuo Wu, and Hongdeng Qiu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02175 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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Analytical Chemistry

Selective Separation of Metal Ions via Monolayer Nanoporous Graphene with Carboxyl Groups Π

Zhan Li †,§, Yanqi Liu£, Yang Zhao , Xin Zhang£, Lijuan Qian£, Longlong Tian¥, Jing Bai§, Wei Qi€, Huijun Yao§,$ ,*, Bin Gao$, Jie Liu§, Wangsuo Wu£, Hongdeng Qiu†,* †

Key Laboratory of Chemistry of Northwestern Plant Resources, Key Laboratory for Natural

Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China §

Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China

£

The School of Nuclear Science and Technology, Lanzhou University, Lanzhou, 730000, China

Π

Department of Chemistry, State Key Lab of Molecular Engineering of Polymers, Shanghai Key

Lab of Molecular Catalysis and Innovative Materials, and Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200433, China. ¥

School of Radiation Medicine and Protection & School for Radiological and Interdisciplinary

Sciences (RAD-X), Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Medical College of Soochow University, Suzhou, Jiangsu 215123, China €

Institute of Applied Electromagnetic Engineering, School of Electrical and Electronic Engineering,

Huazhong University of Science and Technology, Wuhan, 430000, China $

Department of Agriculture and Biological Engineering, University of Florida, Gainesville, Florida 32611, USA *Corresponding Author E-mail: [email protected] (H. Y.), Fax No.: +86-931-4969332 E-mail: [email protected] (H. Q.), Fax No.:+86-931-4968877 1

ACS Paragon Plus Environment


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ABSTRACT: Graphene-coated plastic substrates, such as polyethylene terephthalate (PET), are regularly used in flexible electronic devices. Here we demonstrate a new application of the graphenecoated nanoporous PET membrane for the selective separation of metal ions in an ion exchange manner. Irradiation with swift heavy ions is used to perforate graphene and PET substrate. This process could create graphene nanopores with carboxyl groups, thus forming conical holes in the PET after chemical etching to support graphene nanopores. Therefore, a monolayer nanoporous graphene membrane with a PET substrate is constructed successfully to investigate its ionic selective separation. We find that the permeation ratio of ions strongly depends on the temperature and H+ concentration in the driving solution. An electric field can increase the permeation ratio of ions through the graphene nanopores, but it inhibits the ion selective separation. Moreover, the structure of the graphene nanopore with carboxyl groups is resolved at the density functional theory level. The results show the asymmetric structure of the nanopore with carboxyl groups, and the analysis indicates that the ionic permeation can be attributed to the ion exchange between metal ions and protons on the two sides of graphene nanopores. These results would be beneficial to the design of membrane separation materials made from graphene with efficient online and offline bulk separation.

KEYWORDS: selective separation, ion irradiation, graphene nanopores, metal ions.

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INTRODUCTION Graphene, a two-dimensional (2-D) material with excellent mechanical strength,1-3 has become an ideal candidate material for preparing solid state nanopore.4,5 Several researchers have reported that the proper size of a graphene nanopore could selectively separate gas molecules or inorganic ions.6-18 Moreover, it was confirmed that protons can freely cross a perfect monolayer of graphene without defects.19,20 So graphene has great potential for applications in the separation of hydrogen isotopes, which would push the graphene-based membrane separation to a higher level. However, it is urgent to resolve the issue of how to regulate the size of filter pores in graphene. Some groups utilized a focused or unfocused ion beam to irradiate graphene to regulate the nanopores.21,22 It was found that the focused ion beam could produce relatively large nanopores in graphene (pore diameter >10 nm).23 Meanwhile, Russo et al. utilized a unfocused ion beam to produce nanopores of ~ 0.5 nm on monolayer graphene.24 O'Hern et al. observed the clear shape of graphene nanopores prepared by the bombardment of Ga ions and performed a preliminary investigation of the ion transport properties.25 However, graphene nanopores were always considered to be the rigid planar while investigating the process of ion permeation, and the chemical groups around the graphene nanopores were not treated as the main cause for ion separation and permeation. Therefore, there is still a great gap in the understanding of the permeation and separation of inorganic ions through graphene nanopores. Furthermore, a substrate for supporting filter pores would be indispensable for improving its manufacturability and toughness. Porous graphene must exactly overlay a flat perforated substrate,26 3



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which should not prevent ions from passing through; otherwise, the ions would be blocked by the substrate.27 Meanwhile, Suk et al.26 reported that graphene covering holes of large size was easily to be broken because of surface tension, so it is necessary to regulate the size of holes (< 2 µm) in the substrate. Irradiation of the graphene/polymer film with swift heavy ions could be the perfect solution to satisfy the requirement discussed above.28 Accordingly, graphene with a polymer could be penetrated by energetic ions, leaving nanopores in the graphene and latent tracks in the polymer film. The latent tracks, when etched by a strong base, would be enlarged and acted as supporting holes with a certain size and shape. These pores in graphene would just cover the etched holes of the substrate, and the intrinsic defects (1~15 nm) 29 in graphene would be blocked by the entire polymer membrane. Consequently, the inorganic ions would pass through the pores in the graphene, and they would then freely and quickly pass through the etched holes in the polymer substrate. Therefore, under the influence of chemical groups and the size of the graphene nanopores, these ions would be selectively sieved under osmotic pressure or an electric field.6 Recently, monolayer graphene with a polymer film, such as a polyethylene terephthalate (PET) substrate, has been regularly used in the field of flexible electronic and photonic devices.30-34 However, in this work, we utilized energetic

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Kr25+ ions to bombard monolayer graphene with a

PET substrate to produce porous graphene on a PET substrate with conical holes after chemical etching, and then investigated the ionic separation effect of the obtained monolayer nanoporous graphene with a PET substrate (MNGM-PET) under the driving force of H+ gradient as schemed in Figure 1. Moreover, to analyze the process of ion permeation and selectivity through nanoporous graphene, the electrostatic potential (ESP)-mapped molecular Van der Waals (vdW) surface of the graphene nanopores was also determined by the density functional theory (DFT).35,36 These results 4


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would provide the theoretical basis for the practical application of graphene and promote the industrialization of graphene in membrane separation.

Figure 1. Preparation process of MNGM-PET and separation of related ions. The ion bombardment of monolayer graphene on a PET substrate by Kr ions, and the chemical etching of the PET substrate. The MNGMPET was placed in the separation device for the separation of ions.

EXPERMENTAL SECTION

Transfer of graphene with copper basal membrane to a PET substrate: Monolayer graphene (2 × 2 cm2) with a copper substrate was purchased from JCNANO Co. Ltd. (Nanjing in China, an agency of ACS Material Co., Ltd.), and it was then transferred to a PET membrane according to the method described by Chen et al.37 The details are as follows: the graphene film on Cu foil was drop-coated with polymethyl methacrylate (PMMA, 46 mg/mL, purchased from Sigma-Aldrich), which was then cured at 180 °C for 1 min. The 2 × 2 cm2 × 50 µm thick Cu substrate was then etched away by an aqueous solution of iron nitrate (0.05 g/mL) over a period of ∼24 h. The PMMA/graphene stack was washed with deionized water, and transferred to the target substrate, and dried. After the transfer to the target substrate of PET (4 × 4 cm2 × 20 µm), a small amount of liquid polymethyl methacrylate (PMMA) solution was dropped onto the PMMA/graphene to dissolve the percolated PMMA. The new PMMA film was then slowly cured at 5



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room temperature for approximately 30 min and then dissolved by acetone; the detailed procedures are shown in Figure S1, Supporting Information. Preparation and characterization of MNGM-PET: Bombardment of ions: The single layer graphene membranes on PET and Cu were irradiated at the Heavy Ion Research Facility in Lanzhou (HIRFL), China. To achieve the appropriate irradiation fluence, a scintillation radiation detector was installed behind the sample. With a fluence of 1× 10 ions/cm2, 1900 MeV 84Kr25+ ions were used to irradiate the single PET membrane (20 µm) and the single layer graphene membrane on a PET substrate (20 µm), which were used as filter membrane. The graphene on Cu foil (25 µm) was irradiated with a fluence of 1 × 10 ions/cm2 (1900 MeV 84

Kr25+), which was used for morphology characterization. Characterization: For the X-ray photoelectron spectroscopy (XPS) and the Raman analysis of

graphene after irradiation, a single layer of graphene was transferred to a Si/SiO2 wafer using a sacrificial polymer transfer procedure similar to that described in reference.38 Another bombarded graphene on Cu foil sample was dissolved in iron nitrate (0.05 g/mL) for 24 h and then transferred to a gold grid according to reference.25 This sample was used for the shape characterization of graphene nanopores with TEM (FEI Tecnai F30) and to study the etching effect of the electron beam (150 kV) on the graphene nanopores.24,25 Etching of substrate: The irradiated graphene on a PET substrate and the single PET membrane were mounted in a home-made two-compartment electrolytic cell; 0.05 M HCl was poured into one part of the cell close to the graphene and served as a stopping solution, while 1 M NaOH was poured into the other part close to the PET as the etching solution (Figure 1, Figure S1 and S2). During etching, a voltage (0.1 V) was applied between two Pt electrodes, which were immersed separately in 6


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the etching solution and stopping solution, to monitor the transmembrane current. After the electric current reaching equilibrium, all the pores were opened and the etching produce was stopped by the stop solution. The etchant and stopping solutions were removed, and the samples were immediately washed with 0.1 M HNO3 for three times. Finally, the monolayer nanoporous graphene membrane with PET (MNGM-PET) and the porous PET membrane were removed from the electrolytic cell for shape, size, and structural characterizations of the conical holes via a scanning electron microscope (SEM). Preparation of permeation equipment: The filter equipment with two sinks (V=15 mL) was prepared using Perspex material (Figure S3A). Because the PET membrane with conical nanoholes might affect the ionic transport across the graphene nanopores, the solution containing metal ions was poured into the sink close to the side of the graphene in MNGM-PET, which was referred to as the source sink. In contrast, HNO3 was used as the driving liquid and was poured into the driving sink close to the PET side (Figure S3B). Detailed description about the filter equipment is given in Supporting Information. Effect of organic molecules on the permeation of protons: MNGM-PET was mounted in the filter device firstly. Three groups of 10 mL mixed solutions (pH 1) consisting of ethanol (0%, 4%, and 25%) and HNO3 were ultrasonically treated for 30 min under heating to remove dissolved gases, and then, 5 mL of mixed solution was injected into the source sink with MNGM-PET; at the same time, a neutral water solution (pH 5.0-6.5) was poured into the driving sink. A pH electrode (pH meter, Mettler 405-60-T) was immersed in each of the sinks for the detection of acidity. Under magnetic stirring, the values of pH in the driving solution were taken at any time. Then, 10 µL samples from the driving sink were collected at various times and 7



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diluted to 1 mL for the detection of ethanol using UV-vis spectroscopy (at 240 nm, Lambda25, PerkinElmer). The porous PET membrane was also installed in another device as the control for the above experiment. Separation of inorganic metal ions: Permeation kinetics of K+: Solutions of HNO3 (pH 1) and KNO3 (0.1 mol/L) were ultrasonically treated for 30 min under heating to remove dissolved gases; 5 mL HNO3 was poured into the driving sink in the device mounted with MNGM-PET, and 5 mL KNO3 was poured into the source sink. Under magnetic stirring at 293 K, 10 µL of driving liquid was collected after 5 min and 15 min and then after 1, 4, 8, 14, and 16 h. The aliquots were then analyzed for K+ using ICP-AES (IRIS Advantage ER/S, TJA). The same experiment was repeated at 328 K to study the effect of temperature on the permeation dynamics. The control experiment was performed in the same device with the PET membrane according to the above steps. The separation of metal ions: After ultrasonic degassing, HNO3 (pH 1) and a mixed solution of different metal salts (0.1 mol/L per ion, pH 2.5), such as the monovalent ions LiNO3, NaNO3, and KNO3 and the different valent ions KNO3, Ca(NO3)2, and Fe(NO3)3 or NaNO3, Mg(NO3)2, and Fe(NO3)3, were separately poured into the driving and source sinks in the device mounted with MNGM-PET. Under magnetic stirring at 293 K, the ionic contents of each sink were detected after 16 h. The separation of Fe(NO3)2 and Fe(NO3)3 (0.1 mol/L, pH 2.5) was also studied according to the above methods, and the concentrations of Fe3+ and Fe2+ were detected after 16 h using ion chromatography (861 Advanced Compact IC, Switzerland). The control experiment of porous PET membrane was carried out with the same device according to the above experiment. The effect of the acidity gradient and electric field on the K+ permeation and the permeation of 8


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inorganic anions: After ultrasonic degassing, solutions of HNO3 with different pH were poured into the driving sink, and KNO3 (0.1 mol/L, pH 2.5) was poured into the source sink. The concentrations of K+ in the driving solution were detected after 1 h. HCl (pH 1) and KNO3 (0.1 mol/L, pH 2.5) solutions were poured into the driving sink and source sink, respectively. Afterward, under magnetic stirring, 10 µL of the driving liquid was collected after 16 h from two devices installed with MNGMPET and the nanoporous PET, and the NO3- content of the samples was determined by ion chromatography. Based on the above methods, the filter kinetics of MNGM-PET for K+ and the separation of Li, Na, and K was studied in an electric field generated by a potential difference of 5 V. Geometry and electronic structure of graphene nanopores: The initial structures of graphene, graphene nanopores with –COOH, graphene nanopores with – COOH attracted to Na+, graphene nanopores with –COONa, and graphene nanopores with –COONa attracted to H+ were prepared using wavefunction analysis in combination with visual molecular dynamics (VMD) software. The optimization was realized by DFT.

RESULTS AND DISCUSSION

The preparation of graphene nanopores with ion bombardment: It is reported that swift heavy ion bombardment could create sub-nanometer pores in monolayer graphene,24 but these pores could hardly be observed directly by TEM without spherical aberration correction. Therefore, to confirm the existence of nanopores in graphene, the electron beam (150 kV, TEM) from the TEM was used to etch defective sites in graphene created by the bombardment of swift Kr ions; this would enlarge the defective pores to a certain size as described in references.24,25 It should be noted that the electron beam of low energy could not damage the pristine graphene,24 as 9



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shown in Figure 2A-C. However, two sub-nanometer pores could be observed in the bombarded monolayer graphene in Figure 2D, and these pores (within 0.4-0.6 nm) could be associated with the defective sites.39 After etching by the electron beam for 5 and 10 min, one pore of these pores was enlarged to ~0.8 (Figure 2E) and ~1.6 nm (Figure 2F), respectively. Moreover, 12 diffraction spots are clearly observed in the bombarded graphene, as shown in Figure 2D, which belong to the diffraction pattern of monolayer graphene.40,41 This suggests that we have successfully prepared the monolayer nanoporous graphene membrane, and the size of most pores is close to that of O’Hern et al. (0.4 nm).25

Figure 2. Effect of electron beam etching on graphene nanopores created by Kr ion bombardment. (A) HRTEM of pristine graphene, the etching time is 0 min; (B) HR-TEM of pristine graphene, the etching time is 5 min; (C) HR-TEM of pristine graphene, the etching time is 10 min; (D) HR-TEM of graphene after bombardment with 1×1011 ions, and the white arrows identify the nanopores. The image includes two nanopores marked as No. 1 and 2. The top right corner shows an enlarged image of the No. 1 nanopore, and the blue arrow indicates the TEM diffraction of the bombarded graphene in bottom right corner; (E) HR-TEM of the pores after 5 min of etching with the electron beam, white arrows point to No. 1 nanopore; (F) HR-TEM of pores for 10 min after etching with electric beam, and the white arrows identify the No.1 nanopore.

It is reported that the density of pores on graphene depends on the amount of bombardment ions and the number of sp3 carbon atoms in graphene.2,25,42 The Raman spectroscopy of irradiated 10


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graphene (Figure 3B) shows that the intensity of the D1 band for graphene increased with respect to the pristine sample (Figure 3A). D1 is the characteristic peak for sp3 carbons in graphene,25,42 and its increase in intensity suggests that sp2 disrupted by swift heavy ion domains in the graphene. Moreover, the effect of the bombardment time on the D1 and G bands is shown in Figure 3C-D. It is observed that the integral area ratio of D1/G increased with the increasing of irradiation fluence, indicating that more defective sites were formed in graphene with the increase of the bombardment fluence25,43. Figure 3E-F shows the X-ray photoelectron spectroscopy (XPS) data for bombarded and pristine graphene. The existence of C-C (284.8 eV), C=O (287.6 eV), and C-O (287.1 eV) on bombarded graphene was confirmed, which indicates that sp3 carbon atoms have been oxidized to COOH or –OH by the oxygen in air. Therefore, the monolayer graphene nanopores with oxygen groups can be used as an ion exchange membrane to separate metal ions in accordance with the report of Sint. et al.44 These results proved that the MNGM had been prepared successfully. Then, the chemical etching with NaOH to produce nanopores in the PET substrate connected to the MNGM was studied.

Figure 3. Spectral characterization of bombarded graphene. (A) Raman spectrum of pristine single-layer 11



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graphene; (B) Raman spectrum of bombarded graphene; (C) The effect of bombardment time on D1 and G peak with D1 band at ∼1350 cm−1 and G band at ∼1620 cm−1; and (D) D1/G integral area ratio; (E) C 1S XPS spectrum of pristine graphene; (F) The C 1s XPS spectrum of the bombarded graphene after 4 h.

Preparation of the perforated PET substrate with conical holes: The bombarded PET in contact with the MNGM was placed into the etching equipment (Figure 4A), and then, NaOH was used to etch the nuclear tracks to prepare the tapered holes in PET (Figure 4B). The electric current curves for the etching of the PET in MNGM-PET and the single PET membrane are shown in Figure 4C-D. After achieving a stable etching, the electric current for the single PET membrane is higher than that for MNGM-PET at a special voltage (0.1 V). This could be attributed to the low permeability of ions through the graphene nanopores compared with that through PET. The etched conical pores in PET are characterized by SEM, as shown in Figure 4E-H. The base side and the tip side of the conical pores in PET and the graphene side of MNGM-PET are observed directly; the tip size and base size of the conical pores in the PET membrane range from 10 to 20 nm (Figure 4G) and 100 to 150 nm, respectively. However, from Figure 4H, we cannot see any nanopores on the graphene side of MNGM-PET (except for those of the PET substrate), which may be because the size of the nanopores in graphene is beyond the SEM resolution. Moreover, it must be noted that the pore size in Figure 4E is almost twice the size of that in Figure 4F, suggesting that the presence of graphene may promote etching of nuclear tracks. We speculated that graphene can improve the electrical conductivity through conical holes in PET in the process of etching, but it needs to be further validation. These results from Figures 2-4 indicate that a monolayer porous graphene membrane on a permeable PET substrate had been successfully prepared, which was then wedged between the sinks of the source and driving solution for the study ion separation.

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Figure 4. The etching of PET substrates. (A) Etching equipment; (B) The etching process of MNGM-PET with NaOH; (C) The etching current for the MNGM-PET membrane vs. time; and (D) the etching current of the single PET membrane as a function of time; (E) SEM of the base side of the conical pore on the PET substrate of MNGM-PET after bombardment and etching; the bar is 2 µm; (F) SEM of the big pore side on PET after bombardment, bar is 1 µm; (G) Pore tip side of conical pore in PET after bombardment and etching; the bar is 1 µm; HR-SEM at the top right-hand corn, the bar is 100 nm; (H) SEM of the graphene side of MNGM-PET after etching; the single layer graphene cannot be observed by SEM, and the bar is 2 µm.

The effect of organic molecules on the proton permeation through MNGM-PET: In actual applications, a mixture of organic molecules and inorganic ions often needs to be addressed. However, the size of organic molecules is remarkably larger than that of ions, which means that organic molecules may affect the ion permeation efficiency of the graphene nanopores. The H+ transport process through graphene pores in the presence of ethanol was examined, and the corresponding results are shown in Figure 5. For the PET membrane, in the absence of alcohol, the decreasing trend of pH in the source solution shows transitions from fast to slow as time going on, indicating that H+ permeation through PET membrane exhibits transitions from fast to slow, which is similar to the behavior of MNGM-PET. The presence of alcohol (4% and 20%) lowers the decreasing rate of pH value for the PET membrane, where the decreasing trend is not as high as before, meaning that the presence of alcohol can strongly affect H+ permeation through PET membrane. This is because alcohol molecules (> 1 nm) can enter the conical nanopores in PET 13



together with H+, which increases the resistance of proton permeation and decreases the proton permeation rate. In contrast, the decreasing trend of pH through MNGM-PET is not changed by the alcohol, meaning that the presence of alcohol does not change so much the H+ permeation trend through MNGM-PET; however, as can be seen from the slopes of the curves, the rate of decrease of pH is reduced, indicating that alcohol can inhibit the permeation through MNGM-PET, which can be attributed to the fact that some of the graphene nanopores are blocked by the alcohol molecules. In addition, the presence of ethanol in the driving liquid could not be determined by UV-vis spectroscopy for both the PET and MNGM-PET membranes, indicating that ethanol cannot pass through the graphene nanopores, which is consistent with reference.25 Therefore, it is necessary to remove organics to accelerate ionic separation through MNGM-PET. Meanwhile, the decreasing trend of pH for MNGM-PET is similar to that for the PET, meaning that the rate of H+ permeation through MNGM-PET is very high. Because H+ ions can rapidly permeate through MNGM-PET at lower economic costs in actual applications, they have been used as the driving ions in ion separation experiment.

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The separation of metal ions by MNGM-PET: 14


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The separation kinetics are related to the separation efficiency; therefore, the kinetic process by which K+ permeated through MNGM-PET and PET at 328 and 293 K was first investigated, as shown in Figure 6A-B. The K+ content migrating from the driving solution through MNGM-PET nearly increases linearly with the permeation time, but the content that moved through PET first increases sharply and then stabilized after 15 h. The amount of that K+ permeated through MNGMPET (~180 µmol/L after 24 h at 323 K) is significantly lower than that through the PET holes (~340 µmol/L after 24 h at 323 K), suggesting that K+ can more easily permeate through PET because of its larger pores. Meanwhile, it is found that the permeation amount through the MNGM-PET and PET membranes strongly depended on the temperature in a certain time, suggesting that a high temperature can promote the permeation of ions through MNGM-PET and PET because of the higher ion mobility. The separation of monovalent metal ions (K+, Na+ and Li+) through MNGM-PET and PET is studied, as presented shown in Figure 6C. After 16 h of permeation through MNGM-PET, the remaining K+, Na+, and Li+ contents in the driving solution are ~35, 20, and 15 µmol/L, respectively; however, the contents of these ions after permeation through PET are approximately the same, 59 mmol/L, indicating the lack of ionic selectivity. This suggests that monovalent metal ions can be separated through the use of MNGM-PET. Moreover, K+ exhibits the highest permeation ability, followed by Na+, Li+ with lower permeability. This observation is consistent with the sequence of hydrated ionic radius, 3.82 Å Li+> 3.58 Å Na+> 3.31 Å K+ 45. When the radius of a hydrated ion is larger, its coordination ability with oxygen is weaker.46,47 Accordingly, K+ could coordinate with oxygen groups in an easier manner than Na+ and Li+. Therefore, the K+ in the solution of monovalent ions would pass through MNGM-PET more easily. 15



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It was reported that the permeation sequence of ions strongly depends on their velocity within an electric field in the solution.48 In this regard, the sequence would not only depend on the hydrated ionic radius but also strongly relate to the ion mobility. Therefore, the permeation sequence of K+ > Na+ > Li+ would also be depended on the mobility of ions in an electric field. The mobility of ions would also depend on their ionic valence in the solution. Meanwhile, the mobility and permeation of ions through the graphene nanopores are related to the ionic valence in the solution. Accordingly, the oxygen-containing groups with negative charges decorating the MNGM-PET nanopores could strongly attract cations with different valences. This means that metal ions with different valences would perform differently regarding the permeation results. With this assumption, the effect of the valence on the separation through MNGM-PET under a H+ driving force was studied. KNO3, Ca(NO3)2, and Fe(NO3)3 were set as one group, while NaNO3, Mg(NO3)2, and Fe(NO3)3 were set as another group (all the solutions were 0.1 mol/L with a pH value of 2.5). The contents of K+, Ca2+, and Fe3+ in the driving solution after permeation through MNGM-PET are ~21, 7.2, and 17 µmol/L, respectively; the contents of Na+, Mg2+, and Fe3+ are ~25, 8.5, and 27 µmol/L, respectively; and there is no selectivity for ionic permeation through PET. The results in Figure 6D-E show that the monovalent ions and trivalent ions could pass through MNGM-PET more easily than the divalent ions. In the absence of Fe3+, monovalent ions would be easily separated from divalent ions, and Fe3+ could be separated from divalent ions in the absence of monovalent ions. To further prove this deduction, the separation of Fe2+ and Fe3+ was also studied, as shown in Figure 6F. The contents of Fe2+ and Fe3+ in the driving solution after permeation through MNGM-PET are ~6.2 and 21 µmol/L, respectively, while the PET showed no selectivity. This suggests that Fe3+ could also be separated from Fe2+ by using MNGM-PET. These results show that the filtering ability of MNGM-PET is 16


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strongly influenced by ionic valence. In addition, Fe3+ has a greater charge and a larger hydration radius, and Mg2+ and Ca2+ are lower down in the sequence relative to Na+. These results could be attributed to the combined action of the lower mobility and larger hydrated ionic radius (Tables S1 and S2).

Figure 6. The separation of different metal ions by the porous graphene driven by H+. (A) and (B) The separation dynamics of K+ through MNGM-PET(A) and PET (B) at 293 and 328 K, respectively; (C) G-16h and P16h indicate the concentrations of Li+, Na+, and K+ in the driving solution after permeation through MNGM-PET and PET, respectively; (D) G-16h and P-16h denote the concentrations of K+, Ca2+, and Fe3+ in the driving solution after permeation through MNGM-PET and PET, respectively; (E) G-16h and P-16h represent the concentrations of Na+, Mg2+, and Fe3+ in the driving solution after permeation through MNGM-PET and PET, respectively; and (F) G-16h and P-16h depict the concentrations of Fe2+, and Fe3+ in the driving solution after permeation through MNGM-PET and PET, respectively. In all the experiments, the initial concentrations of the different ions are 1 mol/L, the pH in the driving solution is 1, and the time for the separation experiment is 16 h.

The carboxyl groups around the graphene nanopore would repel anions and inhibit them from passing through the membrane. Thus, we continued studying the permeation behavior of MNGMPET by monitoring NO3- (Figure 7A), and HCl was selected as the driving solution. Figure 7A reveals that NO3- was not detected in the driving solution because of the poor coordination between the anion and oxygen. Moreover, the effect of the H+ concentration in the driving solution on the

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permeation of K+ in Figure 7B demonstrated that a high concentration of H+ could promote the permeation of K+ through MNGM-PET. However, the concentration of H+ would reach equilibrium after a change in pH from 3 to 4. The concentration of H+ in the driving solution for separation using PET remained nearly unchanged and was much higher than that obtained with MNGM-PET. This is due to the free permeation of ions through PET. These results suggest that adsorption and desorption processes involving the ion exchange membrane occur simultaneously on the right and left sides of MNGM-PET during the separation process. On the side of the source solution, the metal ions would interact selectively with the carboxyl groups near the graphene nanopores, which were rapidly replaced by H+ from the other side and then desorbed into the driving solution. Hence, the permeability would strongly depend on the rates of adsorption and desorption and the concentration of acid in the driving solution, which confirms the process of ionic exchange through the graphene pores with oxygen groups. Therefore, if the ∆pH between the driving solution and the source solution can be fixed at a high value, then a high permeation ratio could always be obtained. These results suggest that this method has potential for integration into online separation technologies

Figure 7. (A) Detection of NO3- using ion chromatography; (B) the effect of a pH gradient on the permeation of K+. (A) The peak at ~1.5 h is HCl, and the one at ~2.5 h is HNO3. After permeation of the ions, NO3- could not be found in the sample of MNGM (the filter time is 1 h, and the driving liquid is HCl). (B) The design of cyclic sieve processes and the sieve mechanism of ions through nanopores in graphene. The ion transport through nanopores is similar to ion pumps. 18


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On many occasions, the driving force of an electric field is used to accelerate the membrane separation process, so a potential difference of 5 V was selected to study the electric field effect on ionic separation, and the results are shown in Figure 8. The permeation rate of ions through MNGMPET is improved significantly by the electric field, but we do not observe a good separation of the ions comparing with the control group at 0 V. This suggests that the metal ions driven by an electric field could permeate rapidly through the graphene nanopores, but the ionic selectivity is absent. This could be ascribed to the disappearance of an ionic exchange through the graphene nanopores in the presence of an electric field. Thus, the results also prove that ionic exchange is the main cause of ion separation through the graphene nanopores.

Figure 8. Separation of ions driven by an electric field with a potential of 5 V. (A) The effect of the electric field on the permeation kinetics of K+; (B) the effect of the electric field on the separation of Li+, Na+, and K+.

The simulation of ion exchange through graphene nanopores: The transport of metal ions through graphene nanopores depends on the geometry, electrostatic potential (ESP), and charge distribution of the graphene nanopores. Therefore, the ESP-mapped molecular vdW surface of a graphene nanopore with –COOH and its related processes are successfully resolved. The ESP of a vdW surface has been frequently used to study reactivity,35,49 but 19



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from an ESP point of view, it is also used to study the ion transport through nanopores. The site possessing a more negative ESP has a stronger ability to attract cations than a site with a more positive ESP. In Figure 9, for the graphene (Figure 9A1 and A2) unit region, it can be seen that the ESP is evenly distributed across the surface of graphene (negative ESP) in addition to the edge region (positive ESP). Furthermore, it shows the 2-D geometry with bilateral symmetry. However, in the ESP map of the nanopores, the 2-D graphene structure has converted into a 3-D geometry because of the existence of chemical groups around the nanopore (Figure 9B-E); it is seen that – COOH protrudes from the plane of graphene and causes the structure to take on a left-right asymmetry (Figure 9B2-E2).

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Figure 9. ESP-mapped molecular vdW surface of graphene and graphene nanopores, and the area percent in each ESP range. The red region represents negative ESP, while the blue region represents positive ESP. The unit is kcal/mol. (A1-2) and (B1-2) are the structures of graphene and a graphene nanopore with –COOH. (C1-2), (D1-2), and (E1-2) are of the ion exchange processes.

CONCLUSIONS We have successfully fabricated a monolayer nanoporous graphene membrane with a PET substrate using an ion irradiation technique. The sp3 carbons of the bombarded nanopores in graphene are oxidized to carboxyl groups, which could be used to rapidly separate metal ions from solution. Meanwhile, organic molecules do not pass through the nanopores; however, they decrease 21



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ionic transport through the graphene pores. For the same radius and charge, the ionic permeation ratio depends strongly on the concentration of the driving ions. The separation effect can be attributed to an ionic exchange facilitated by oxygen-containing groups around the nanopores of graphene. While an electric field promotes the permeation ratio of ions through graphene nanopores, but it will decrease the ion selectivity. Moreover, the ESP-mapped molecular vdW surfaces of graphene and graphene nanopores with carboxyl groups are resolved and confirms that ion exchange is the main mechanism of the ion selective separation through nanopores.

SUPORTING INFORMATION Supporting Figure S1-S4, Table S1-S3 and other experimental data is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Authors [email protected] (H. Y.); [email protected] (H. Q.). Acknowledgments This study was conducted with financial support from the National Natural Science Foundation of China (No. J1210001, 31201841, 11575261, 11275237, 21405165), the Funds for Distinguished Young Scientists of Gansu (1506RJDA281) and the top priority program of “One-Three-Five” Strategic Planning of Lanzhou Institute of Chemical Physics, CAS.

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Selective Separation of Metal Ions via Monolayer Nanoporous

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