Revealing the Role of Oxygen Debris and ... - ACS Publications

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C: Physical Processes in Nanomaterials and Nanostructures

Revealing the Role of Oxygen Debris and Functional Groups on the Water Flux and Molecular Separation of Graphene Oxide Membrane: A Combined Experimental and Theoretical Study Dae Woo Kim, Jidon Jang, In Kim, Yoon Tae Nam, Yousung Jung, and Hee-Tae Jung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03318 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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The Journal of Physical Chemistry

Revealing the Role of Oxygen Debris and Functional Groups on the Water Flux and Molecular Separation of Graphene Oxide Membrane: A Combined Experimental and Theoretical Study Dae Woo Kim†,§,*, Jidon Jang§,‡, In Kim‡, Yoon Tae Nam†, Yousung Jung‡,*, Hee-Tae Jung†,* †

Department of Chemical and Biomolecular Eng. (BK-21 plus) & KAIST Institute for Nanocentury, Korea

Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea ‡

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of

Science and Technology, Daejeon 34141, Republic of Korea §

D. W. Kim and J. Jang contributed equally to this work.

*Corresponding authors: D. W. Kim ([email protected]), Y. Jung ([email protected]), and H.-T. Jung ([email protected]) ABSTRACT: A combined experimental and molecular dynamics study revealed the role of oxygen debris (ODs) and functional groups on the nanofiltration performance of a graphene oxide (GO) membrane. A NaOH treatment removed ODs adsorbed onto the graphene oxide (DGO). COOH-decorated graphene oxide (CGO) was prepared by controlling the oxidation time of Hummer’s method, resulting in 45% of carbons with a COOH group. The water permeance of the prepared graphene oxide membrane without ODs was one order of magnitude greater than that of graphene oxide membranes in our experiments: 1.24, 1.59 and 14.7 L m-2h-1bar-1 for GO, CGO and DGO. However, the rejection of dye molecules below 1 nm in size was dramatically reduced without ODs, indicating that ODs play a critical role in rejecting molecules below 1 nm in size due to electrostatic and hydrogen bonding interactions and by narrowing the effective interlayer spacing, as noted in molecular simulations. Additionally, COOH-decorated graphene oxide membrane displayed similar separation performance compared to common graphene oxide membrane mainly decorated with OH and epoxy.

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1. INTRODUCTION Laminated graphene oxide membranes have been widely applied for water treatment processes such as nanofiltration1, desalination2, forward osmosis3 and pervaporation4. The application of these membranes is attributed to the precise and fast molecular separation by the molecular sieving effects and electrostatic interaction between the molecules and functional groups of the graphene oxide5. The excellent chemical and mechanical properties of graphene oxide film are also beneficial for practical applications under harsh operating conditions, such as those involving high pressure levels and chemical solvents2,6. In addition, graphene oxide membranes can easily be mass-produced on ceramic or polymeric supports via solution processes such as spin coating, bar coating, spray coating and vacuum filtration1,2,3,7. The structure of the graphene oxide membrane has been modified to enhance the water flux and rejection of salt ions and dye molecules, which could be helpful to reduce the plant operation cost of practical applications such as desalination and nanofiltration8. Because the flux of water molecules through the interlayer of a graphene oxide membrane is dramatically hindered by the narrow interlayer spacing and strong hydrogen bonding between water and oxygen functional groups

9,10,11,12

, additives such as carbon nanotubes13,14, metal

hydroxide nano-strands15, diamines4, and graphene quantum dots16,17 have been utilized to provide enlarged channels. Nano-pores generated in the basal plane of graphene oxide can decrease the diffusion length of water by providing additional entryways for pores in the graphene sheet18. On the other hand, charged molecules (zwitterionic polymers, Ca2+/deoxycholate) have been added to the interlayer or surface of the membrane to enhance the rejection of ions and dye molecules based on electrostatics19. In addition, methods which include the use of a spacer (porphyrin)20, thermal annealing21, and chemical reduction22 can enhance the molecular separation by reducing the effective interlayer spacing of stacked graphene sheets. Physical confinement with a polymer also served to achieve a high rejection rate of monovalent ions, as polymers surrounding interlayer channel prevent the expansion of the interlayer spacing via intercalated water23. Recently, we demonstrated that laminated graphene nanoribbons can improve both the water flux and rejection characteristics due to the extremely reduced diffusion length and abundant charged functional groups at the edges of these types of nanoribbons24. While many experiments and computational simulations have been conducted to improve the flux and rejection of graphene oxide membranes and enriched our knowledge of the mechanisms associated with these membranes, the underlying nature remains elusive. For example, because graphene oxide prepared by chemical oxidation is decorated with oxygen debris (ODs), small and highly oxidized aromatic fragments adsorbed on graphene surfaces25-27, assuming graphene oxide as a plane of graphitic carbons decorated with oxygen functional groups, as was done in previous investigations, may be an oversimplification. Recent research has found that ODs can significantly influence properties such as the solubility28, fluorescence29, catalytic activity30, inherent electro-chemical activity31 and metal particle anchoring25 of graphene oxide, raising important questions about the role of ODs on the performance of graphene oxide membranes. Moreover, because ODs also have a graphitic plane and are electrically charged by oxygen functional groups26,27, the performance of graphene oxide membranes is suspected to be influenced significantly by the steric hindrance and electrostatic interaction between molecules and ODs.


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Herein, we investigate the role of ODs on the pressure-driven filtration performance of graphene oxide membranes in comparison with OD-free graphene oxide and graphene oxide membranes decorated only with COOH. For the removal of ODs adsorbed onto graphene oxide, we utilized a NaOH treatment26,27. The COOHdecorated graphene oxide (44.9 % COOH group) used in this study was prepared by controlling the oxidation time. We also studied the influence of the ODs and functional groups on the water flux of graphene oxide membranes by molecular dynamics (MD) simulations and density functional theory (DFT) calculations to understand the origin of the different permeation behaviors within graphene oxide, graphene oxide without ODs, and COOH-dominant graphene oxide membranes.

2. METHODS Graphene oxide synthesis: Graphite (FP 99.95 % pure, Graphit Kropfmühl AG) was oxidized using a modified Hummer’s method. Graphite (1 g) was mixed with concentrated sulfuric acid (98 %, 150 mL) and 3.5 g of KMnO4 was then added to the solution. After desired times ranging from 1 h to 20 h at 35 °C, distilled water and hydrogen peroxide were added sequentially to the solution in an ice bath. During the oxidation process, the cap of the flask was sealed with Al foil to avoid exposure of the reaction solution to a humid environment. The solution was filtered and the precipitate was washed several times with hydrochloric acid (10 %). The oxidized graphite was freeze dried. Deoxygenation of graphene oxide: The deoxygenation of graphene oxide was conducted by a previously reported method using NaOH26, 27 as a reduction chemical. GO (100 mg) was redispersed into H2O (100 mL) by sonication and NaOH at amounts identical to those of GO was added (100 mg). The solution was then heated to 70 °C for 2 h in an autoclave. The resultant dark solution was centrifuged at 15000 rpm for 60 min. A supernatant solution containing oxygen debris and NaOH was removed. The remaining deoxygenated graphene oxide was redispersed in water via sonication. Fabrication of GO membrane: Graphene oxide was dispersed in DI water at a concentration of 0.1 mg/ml. The dispersions with the desired amounts were diluted in 100 ml of DI water to provide enough filtration time to fabricate the film. Track-etched polycarbonate (PC, pore size: 200 nm, Whatman) was used as a support. Around 4 hour was required to filter the solution and to dry the as-prepared membrane. The prepared membranes were dried at room temperature for at least 5 h. Characterization: Scanning electron microscopy (SEM) and focused ion beam (FIB) techniques were conducted using Quanta 3D FEG (FEI) and field emission SEM (Nova 230, FEI). Raman spectra were obtained via dispersive-Raman spectroscopy (ARAMIS, Horiba Jobin Yvon) using laser excitation at 514 nm. Functional groups of graphenes were investigated using X-ray photoelectron spectroscopy (K-alpha, Thermo VG Scientific). Transmittance electron microscopy (TEM) investigations were performed using a Titan Double-Cscorrected TEM instrument (Titan Cubed G2 60-300, FEI). The Fourier transform infrared (FT-IR) spectra were measured using ALPHA FT-IR spectrometer (Bruker). X-ray diffraction (XRD) patterns were obtained from 5 to 40° by a normal powder X-ray diffractometer (Rigaku, D/MAX-2500) with Cu Kα (λ = 1.5406 Å) radiation (40 kV, 300 mA). The lateral size of the graphene oxide was observed using an optical microscope (OM, LV100POL, Nikon). The contact angle was measured with optical tensiometer (Dyne Technology) and automatically derived by the Young-Laplace equation.



Measuring the filtration performance: All experiments were performed with custom-made dead-end filtration equipment operated at room temperature (~ 25 °C) and with the pressure controlled by the pressure of nitrogen gas (Figure S1). After placing membranes on metal mesh, feed side of membrane was sealed with an O-ring and wetted with feed solution before applying pressure. Generally, the rejection of the filtered solution was realized by analyzing 10 mL of the permeated solution to avoid concentration polarization of the molecules on the surfaces of the membranes by long-term filtration given our use of a dead-end filtration system. Permeance (L m-2·h-1·bar-1) =

Vp t A ∆P

Permeation was calculated based on the formula above. Vp denotes the volume of the permeate solution, t is the permeation time, A is the effective area of the membrane and ∆P is the nitrogen pressure. Rejection (%) =

Cf - Cp Cf

× 100(%)

Rejection rates of each of the dye molecules were calculated by measuring the absorbance of the relevant peaks using an ultraviolet-visible (UV-Vis) spectrometer (Jasco V-570 UV/Vis spectrophotometer) based on BeerLambert Law. The concentrations of the salt ions were measured using an ion conductivity meter. Single-binding energy calculation in DFT: We used Vienna ab-initio simulation package (VASP)32 to calculate the single-binding energy with the revised Perdew-Burke-Ernzerhof (RPBE)33 functionals and dispersion correction by the Grimme D3 method34. A plane-wave basis set and the projector-augmented wave (PAW) method with a cut-off energy of 500 eV were used. Geometries were fully optimized until the force was less than 0.05 eV Å-1. All energies were sampled at the gamma point. The binding energy calculations in this system were not sensitive to k-sampling. The periodic box boundaries of the system were approximately x = 17.21 Å, y = 17.33 Å, and z = 25 Å. Molecular dynamics simulation: For all molecular dynamics (MD) simulations, a reservoir model (Figure S2) was used to create water flows of equilibrated density levels under 2D confinement, and a periodic boundary condition was applied. The sizes of the horizontal GO membranes were in all cases approximately 62.48Å × 62.72Å. The positions of all carbon atoms on the surface of both membrane and ODs (except those of -COOH and –C=O groups) were frozen while functional groups on both were flexible during the MD simulation. All MD simulations were performed using the LAMMPS package35 with a quantum-mechanics-based force field (QMFF-Cx)36 for graphitic carbons. The Dreiding force field37 was used for the functional groups. The SPC/E water model38 was used for the water molecules. The electrostatic potential atomic charges (Table S1) were calculated using Q-Chem, a quantum computation package, by optimizing the circumcoronene structure with each functional group with the B3LYP method, the 6-31+g* basis set and with D3 dispersion correction. The atomic partial charges of the functional groups were applied to the force field. The charge levels of the graphene carbon atoms were set to zero, except for the anchoring carbons of the functional groups. The net charge of the entire system was zero. Four steps comprised the MD simulations in this study. First, the entire system was minimized using steepest descent and conjugate gradient algorithms. Second, the system was heated to 298 K (room temperature) over 0.2 ns in a NVT ensemble with a Nosé–Hoover thermostat. After heating, an equilibrium simulation of the NPT


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dynamics with constant pressure and temperature levels (298 K, 1 atm) was performed for 2 ns, leaving the channel full of water molecules from the reservoir part of the system. Finally, we performed non-equilibrium simulations by applying force (0.03~0.21kcal/mol·Å) to each water molecule in the equilibrated channel to ensure that the net flow rate moved in one direction in the NVT ensemble. The pressure was calculated using the applied force and cross-sectional area of the GO channel.

3. RESULTS AND DISCUSSION

Figure 1. (a)–(c) Schematic illustrations of the preparation process of graphene oxide with controlled functionalization: deoxygenated GO (DGO), GO and COOH-decorated GO (CGO), respectively. (d) Photographic images of each graphene oxide membrane. Insets are schematics of the cross-sectional interlayer structure of each membrane. (e) Top and cross-section SEM images of a graphene oxide membrane. Figures 1a–1c illustrate the approach used here to prepare graphene oxide (GO), graphene oxide without ODs (deoxygenated GO, DGO) and COOH-decorated graphene oxide (CGO). Briefly, the GO was prepared by the conventional Hummer’s method using KMnO4 as an oxidizing agent in a H2SO4 solution18. The 2 h oxidizing reaction synthesized common GO which contained several oxygen functional groups such as OH and epoxy on the basal plane, and COOH groups at the edge. To remove the ODs from the GO, NaOH powder was mixed with the GO solution in the liner of an autoclave and this was then annealed at 70 °C for 2 h, resulting in a dark solution (Figure S3). It is known that a high pH weakens the adhesion of ODs on the surface of GO due to the enhanced repulsion caused by the deprotonation of COOH groups26,27,39. Because excessive oxidation can convert a ketone group into carboxylic acid by forming the O-protonated forms of permanganate oxidizes alkenes in acid40, a long oxidation time (more than 20 h in our experimental conditions) was used to prepare GO with abundant COOH groups (CGO), as shown in Figure 1c. Sonication can easily disperse each graphene material in water due to the negatively charged surface (Figures S3 and S4). The lateral sizes of each GO



sample were mainly micrometer-scale and GO sheets with a size of several tens of micrometer were often observed. While we expect that the structural integrity of CGO sheets could be more destroyed than GO due to the excessive oxidation, the graphene sheets with a size of micrometer scale can cover the entire surface of polymer support without defective holes or cracks. Figure 1d displays photographic images of each membrane. These membranes were fabricated by filtrating the dispersions with track-etched polycarbonate (PC) filter (Whatman, 0.2 um pore size). The GO and CGO membranes were typically a sharp hue of brown. The color of the DGO membrane became slightly dark, possibly due to the partial reduction of oxidized carbons during the NaOH treament26,27. Insets show schematics of the interlayer structures of the GO, DGO and CGO membranes. While there are ODs in the interlayer of GO and CGO, as indicated by the red boxes in the schemes, the interlayer of DGO is composed only of a sp2 carbon region and an oxidized carbon region. Figure 1e presents scanning electron microscopy (SEM) images of the prepared GO membranes, showing a top view and cross-section view. The thickness of all GO membranes was approximately 20 nm so as to mitigate the influence of the membrane thickness on the permeance and rejection1, as shown in the cross-section SEM image (bottom image of Figure 1e). To prepare the cross-section sample, the Pt coating was deposited to protect the membrane from exposure to the ion beam during the focused ion beam technique. Owing to the ultra-thin thickness of the membrane, at approximately 20 nm, the underlying pores of the PC support can be observed in the top-view SEM image. Atomic force microscopy (AFM) results reconfirm the 20-nm thickness of the membrane with a smooth surface for which the RMS value is less than 5 nm (Figure S5).

Figure 2. (a)–(c) Water contact angle images of GO, CGO, and DGO membranes and corresponding XPS C1s spectra. The functional groups and hydrophilic properties of each graphene membrane were investigated with water contact angle measurements and X-ray photoelectron spectroscopy (XPS), as indicated in Figure 2. Figures 2a and 2b show that the GO and CGO membranes had similar contact angles ranging from 35° to 37°, indicating the hydrophilic nature of membranes due to the oxygen functional groups4, 40. Because the NaOH treatment did


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not remove oxygen functional groups completely from the basal plane of GO, the hydrophilicity of the DGO membrane was maintained more like GO and CGO rather than usual reduced GO, showing a contact angle of 40°. The XPS C1s spectrum as obtained from the GO membrane displays peaks corresponding to the oxygen functional groups of C-O (38%) and C=O (13.8%) at 286.5 eV and 287.8 eV, respectively (the graphs on the right in Figure 2), indicating that the GO was dominantly consisted of OH and epoxy groups rather than COOH groups41, 42. On the other hand, the C-C bonding ratio of DGO increased slightly from the rate of 48% noted for GO to 50%, while the C-O and C=O bonding ratios decreased from the rates of 38% and 13.8% for GO to 32% and 9.5%, respectively. The TGA spectra also show suppressed thermal decomposition of DGO as compared to GO (Figure S6). The weight of GO was dramatically decreased at 120°C due to spontaneous decomposition of oxygen functional groups. These XPS and TGA results indicate the decreased amounts of oxygen groups after NaOH treatment possibly attributed to the removal of ODs from GO and partial reduction of GO. The results for CGO demonstrate that 44.9% of carbon was transformed into the COOH form (289 eV). In addition, the XPS C1s spectra show that the proportion of C-O and C=O bonding begins to decrease and that the degree of COOH bonding begins to increase after 10 h of oxidation (Figure S7). Finally, all oxygen groups on the GO are transformed into COOH groups after 20 h of oxidation. The Fourier transform infrared (FT-IR) spectra also verify that the intensity levels of the stretching modes of the O-H (1151 cm-1) and C=O (1724 cm-1) bonds from the COOH groups become considerable after 10 h of oxidation, compared to the intensity of C=C (1610 cm-1) (Figure S8). Thus, the formation of COOH groups can be attributed to the conversion of epoxy and OH groups into COOH groups through the generation of an O-protonated form of an oxygen group during long-term oxidation in addition to the creation of more defective holes or cracks on the basal plane29, 41. We want to emphasize that the oxidized carbons in GO is not significantly reduced to sp2 carbon by the NaOH treatment, which is critical to prepare not reduced GO but deoxygenated GO. As it is known that the increased density of sp2 carbon domain by chemical reduction increases the D over G band ratio42, similar D over G band ratio of DGO and GO indicates insignificant reduction of GO by NaOH treatment (Figure S8d). Based on these characterization outcomes, we conclude that graphene oxide with a different chemical structure was successfully prepared.



Figure 3. (a)–(c) HR-TEM images of GO, CGO and DGO and corresponding inverse fast Fourier transform (IFFT) images, showing the corresponding hexagonal reciprocal patterns in the insets. The white regions in the IFFT images clearly indicate sp2 carbon regions. Dark regions are amorphous oxidized carbon regions. In order to visualize and compare the atomic structures of the GO, CGO, and DGO sheets directly, we conducted high-resolution transmittance electron microscopy (TEM) studies with a Titan Double-Cs-corrected TEM (Titan Cubed G2 60-300, FEI), as indicated in Figure 3. Each graphene sample was highly diluted in water at a concentration of 0.001 mg/ml, and around 50 µL of the dispersions were dropped onto 300-mesh holey carbon grids and dried at room temperature for overnight for the TEM observations. As previously reported43, GO sheets are composed of both graphitic sp2 carbon and oxidized carbon regions; here, while the sp2 region appeared to be smooth, the oxidized region was protruded (Figure 3a). Hexagonal carbon arrays were clearly observed in the sp2 regions, but oxidized carbons are amorphous due to the oxygen functional groups and defective structures. As expected, the CGO sheet better displays the oxidized characteristic of a graphene sheet compared to the GO and DGO sheets because the excessive oxidation induced more defective structures, including atomic pores and abundant oxygen functional groups (Figures 3b and 3c). To clarify the area of the sp2 carbon regions, we conducted inverse fast Fourier transform (IFFT) analyses of each TEM image in Figure 3. Insets show FFT images corresponding to each TEM image, depicting the sharp hexagonal patterns obtained from the sp2 carbon region and the patterns used for the IFFT analyses. Clearly, the area of the sp2 carbon region (shown in white) was increased in the order of CGO, GO and DGO. Because NaOH treatment


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removes the adsorbed ODs on the basal plane, larger sp2 carbon region was observed in DGO compared to GO and CGO25-27. These TEM observations reconfirm the successful preparation of GO with different degrees of oxidation and different chemical structures via NaOH treatment and excessive oxidation.

Figure 4. XRD patterns of GO, CGO, and DGO before (a) and after water swelling (b). The numbers are interlayer distance of membranes. Because the GO membrane rejects molecules mainly by interlayer sieving along with electrostatic interaction between molecules and functional groups on the graphene plane1-5 and the interlayer spacing can be influenced by the molecular structure of GO sheet, the interlayer distance of GO, CGO, and DGO membrane were investigated by X-ray diffraction (XRD) (Figure 4). To dry the membranes, the membrane films were placed in an oven at 60 °C for two days to ensure the evaporation of intercalated water molecules. To swell membrane with water, we immersed the membranes in water for one day and scooped them immediately before the XRD investigation. The interlayer distance of the GO membrane was 9.04 Å, showing a 2θ diffraction peak at 9.77°, which is the typical interlayer distance of GO synthesized by Hummer’s method (Figure 4a)40. Because the NaOH treatment washes the ODs, the interlayer distance of the DGO membrane decreased to 8.19 Å, showing a 2θ diffraction peak at 10.79°. The interlayer distance of the DGO membrane was still larger than that (at somewhat less than 4 Å) of the graphene oxide membrane reduced by hydrazine, hydroiodic acid and thermal annealing due to the remaining oxygen functional groups10,21,26,27. The interlayer distance of the CGO membrane increased from 9.04 Å for GO to 11.05 Å for CGO. Because we dried all membranes at 60 °C for more than two days to remove the intercalated water, the larger interlayer distance of CGO can be attributed to the generation of COOH groups on the basal plane, which are larger than other functional groups, including those of OH and epoxy. The XRD patterns of GO depending on the oxidation time reconfirm that interlayer distance only begins to increase after 10 h of oxidation when COOH begins to form (Figure S9a). After swelling the membrane with



water, the GO and CGO membranes show interlayer distance of approximately 13 Å intercalated with four water layers (Figure 4b). In addition, the water swelling of GO was saturated at the interlayer spacing of around 13 Å regardless of the type of functional group (COOH dominant or OH and epoxy dominant) and the degree of oxidation (Figure S9b). On the other hand, two water layers form in the interlayer of DGO with a interlayer distance of around 9.52 Å. The hindered intercalation of water molecules in DGO can be attributed to the narrow interlayer spacing or the enhanced π-π interaction between adjacent DGO sheets due to the absence of ODs26, 27.

Figure 5. (a) Water flux of GO, CGO and DGO membranes with an increase in the hydraulic pressure. (b) Rejection performance outcomes of GO, CGO, and DGO membranes with several dye solutions (10 mg/L). (c) 0.02 M NaCl rejection with GO, CGO and DGO membranes. The numbers indicate the temperatures (°C) at which the NaOH treatment was conducted. All rejection tests were conducted at 5 bar. (d) Schematic illustration describing the rejection and water flow within the GO nanochannel depending on the presence of ODs. Figure 5 shows the performance outcomes of the GO, CGO and DGO membranes on the filtration test. First, the pure water flux outcomes of each membrane with a thickness of 20 nm were compared while increasing the hydraulic pressure to 10 bar (Figure 5a). All membrane tests were conducted with custom-made dead-end filtration equipment, with the hydraulic pressure controlled by the pressure of the nitrogen gas used. Because GO membranes thicker than 50 nm did not show significant water flow even after filtration tests lasting several hours in our experimental conditions (Figure S10)24, we set the thickness to 20 nm for the membrane test. The low water flux of thick GO membrane could be attributed to the tortuosity of stacked GO sheets1 and low porosity (

Revealing the Role of Oxygen Debris and ... - ACS Publications

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