Article pubs.acs.org/cm
Facile Preparation of Graphene Oxide Membranes for Gas Separation Chenglong Chi, Xuerui Wang, Yongwu Peng, Yuhong Qian, Zhigang Hu, Jinqiao Dong, and Dan Zhao* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore S Supporting Information *
ABSTRACT: We herein report a facile preparation of graphene oxide (GO) membranes including three steps: (1) mild freeze−thaw exfoliation to get large GO nanosheets, (2) purification of exfoliated GO nanosheets through pH adjustment, and (3) spin coating to fabricate smooth GO membranes with uniformly aligned GO nanosheets. The fabricated GO membranes are subject to single gas permeation tests, with the obtained gas permeance in the order He > H2 ≫ CH4 > CO2 > N2 ≫ SF6, indicating a dominant molecular sieving separation mechanism. The H2/CO2 mixed gas permeation tests reveal H2 permeance up to 3.4 × 10−7 mol/(m2·s·Pa) and a H2/CO2 separation factor up to 240, which are among the best of all the reported membranes for H2/CO2 separation. The separation factor drops to 47 at a higher temperature of 120 °C, but the H2 permeance is further increased to 6.7 × 10−7 mol/(m2·s·Pa), ensuring a higher gas separation throughput under higher temperatures. This study paves the way toward large-scale production and application of GO membranes as promising gas separation materials.
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INTRODUCTION Conventional separation methods heavily used in the petrochemical industry are increasingly challenged by energy cost, performance limitation, and environmental concern. Membrane-based separation has attracted much attention owing to its features such as easy operation, low cost, and environmentally benign process.1−3 As a result, it has been widely used in gas separation for clean energy and environmental sustainability, such as H2 purification and precombustion CO2 capture (H2/CO2 separation),4 natural gas upgrading (CH4/ CO2 separation),5 and postcombustion CO2 capture (CO2/N2 separation).6 One of the greatest challenges for membrane separation is to design and develop high-throughput membranes with high selectivity. Commonly used polymeric membranes suffer from the compromise between gas permeability and selectivity, which can be clearly demonstrated by the Robeson upper bounds.7,8 Recently, there has been growing interest in using two-dimensional (2D) layered materials to fabricate ultrathin membranes for gas separation.9,10 These layered materials feature strong in-plane chemical bonds but weak interlayer interactions such as van der Waals forces that can be easily overwhelmed yielding multiple or even single layered nanosheets.11 As a result, they have the ability to exert nanometer control over membrane thickness, affording ultrathin membranes with minimized transport resistance and maximized gas flux. Among all the 2D layered materials, graphene oxide (GO) is receiving increasing attention due to its easy availability and small pore size that can be used to separate gases based on a molecular sieving mechanism.12−15 For example, Choi et al. demonstrated selective gas transport through few-layered graphene and GO membranes.12 They have achieved high CO2/N2 selectivity © XXXX American Chemical Society
using GO membranes under high relative humidity, which is suitable for postcombustion CO2 capture. Yu et al. presented an extraordinarily high H2/CO2 selectivity of 3400 in a 1.8-nmthick GO membrane, which is the highest record of all the membranes reported so far.13 Park et al. reported few-layered GO membranes exhibiting CO2-selective and permeable behaviors under wet conditions, which are promising in natural gas upgrading and CO2 capture.14 Jin et al. proved fast and selective gas-transport channels of laminar GO nanosheets in GO-containing mixed matrix membranes, with a CO 2 permeability of 100 Barrer and CO2/N2 selectivity of 91 suitable for practical CO2 capture.15 Despite this progress, most of the reported GO membranes are targeted toward liquidbased separation such as water treatment and desalination,16 while there are still very limited reports of GO membranes used for gas separation. During our attempts to prepare GO membranes for gas separation, we have identified two key steps: (1) to obtain large and defect-free GO nanosheets with uniform size and (2) to align GO nanosheets evenly onto a porous substrate during membrane fabrication. Herein, we report our finding that can fulfill the above two requirements to consistently prepare high-quality GO membranes with excellent gas separation performance.
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RESULTS AND DISCUSSION Preparation of Uniformly Large GO Nanosheets. We first focus on the preparation of uniformly large GO nanosheets. Like its graphene counterpart, large GO nano-
Received: November 17, 2015 Revised: April 14, 2016
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DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials Scheme 1. Freeze−Thaw Exfoliation of Graphene Oxide
sheets can be prepared by the so-called “Scotch tape” method,17 which however is hard to scale up. Instead, liquid exfoliation through sonication can afford GO nanosheets with large quantities.18 Nevertheless, sonication usually causes the fragmentation of GO nanosheets into smaller pieces with a wide distribution of nanosheet size, which is detrimental to the fabrication of membranes and their performance. Therefore, an alternative approach is needed to prepare uniformly large GO nanosheets in large quantities. It is noted that GO is highly hydrophilic and therefore can easily accommodate water molecules into its interlayer region.19,20 Talyzin et al. observed gradual contraction of GO under different temperatures when bulk water was frozen, which was attributed to the partial withdrawal of water molecules from the interlayer region of GO.21 In addition, the interlayer distance could be restored when the frozen sample was thawed. By making use of this behavior, Ogino et al. demonstrated a facile exfoliation of GO into uniformly large nanosheets with minimum fragmentation by six repetitive freeze−thaw cycles with a fast phase change rate (>40 °C/h).22 This freeze−thaw exfoliation method was adopted by Chakravarty and Latein preparing nanosheets of transition metal dichalcogenide.23 Inspired by these findings, we improved the method using a faster thawing speed (in boiling water bath) to generate irreversible changes between contraction and expansion of GO hoping to get better exfoliation results (Scheme 1). First of all, bulk GO powders were dispersed into water to form an aqueous suspension with a concentration of 0.05 wt % (Figure 1a), which was fastsolidified in a liquid nitrogen bath for 3 min followed by a fast thaw in boiling water bath for 3 min. The freeze−thaw cycle was repeated six times, and the GO suspension became almost homogeneously brown (Figure 1b−g). The obtained GO solution was then centrifuged at 3000 rpm for 5 min to remove large particles. The resulting brown solution exhibits a Tyndall effect due to light scattering of exfoliated GO nanosheets in the solution (Figure 1h). The concentration of GO solution determined by UV−vis is about 0.4 mg/mL (Figure 1i), which equals about 80% of the total GO originally dispersed in the suspension and confirms the effectiveness of this freeze−thaw exfoliation method. In order to obtain the direct proof of GO nanosheets by the freeze−thaw method, the GO solution was dropped onto a silicon wafer and analyzed by atomic force microscopy (AFM) using the tapping mode. As can be clearly seen from Figure 2a− d, GO has been exfoliated into nanosheets with large lateral dimensions up to around 15 um. The height profiles reveal that the exfoliated GO nanosheets have a fairly flat, smooth terrace
Figure 1. (a−g) Optical images of aqueous GO solutions after 0−6 cycles of freeze−thaw exfoliation; (h) the Tyndall effect of exfoliated GO solution; (i) UV−vis spectra of exfoliated GO solutions with different concentrations. The inset shows the linear relationship between GO concentration and UV−vis absorbance at 230 nm.
with a uniform thickness of around 2 nm (Figure 2e,f). Considering that the thickness of a single layer GO nanosheet is around 0.78 nm,24,25 the exfoliated GO nanosheets in this study contain two to three layers of GO, which serve perfectly as building blocks for ultrathin membranes. We have noticed that there are still some very small GO particles mixed with exfoliated GO nanosheets even after highspeed centrifugation. These small GO particles need to be removed; otherwise they may work as “pinholes,” jeopardizing the membrane performance. It has been reported that the solubility of GO nanosheets can be modified by adjusting the pH value of solution, where lower pH value leads to lower solubility of GO nanosheets.26 Following this procedure, we can easily purify the exfoliated GO nanosheets by pH adjustment. To be specific, the pH value of aqueous GO solution obtained after freeze−thaw exfoliation was adjusted to 3.0 with 1 M HCl. After 30 min of standing, the upper half part of the solution was decanted, and the bottom half was diluted with pure water and further treated similarly. This operation was repeated five times, and most of the small GO particles can be removed as revealed by the AFM images (Figure S1). The exfoliated and purified GO nanosheets were further characterized by transmission electron microscopy (TEM, Figure S2), from which the thin and curled GO nanosheets can be easily identified confirming the effectiveness of our exfoliation and purification methods. Dynamic light scattering (DLS) was used to study the size of GO nanosheets before and after the purification process. Although the particle size determined by B
DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
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size of GO nanosheets obtained after purification is larger than the size before purification (13 μm vs 9 μm), indicating a successful removal of small particles. Fabrication of GO Membranes. So far, we have successfully obtained large and defect-free GO nanosheets with uniform size. The next step is to deposit them onto porous supports such as anodic aluminum oxide (AAO) to get membranes for gas separation tests. Recently, there is a growing realization that the deposition method will not only determine the process of membrane fabrication but also directly affect the physicochemical properties of the prepared membranes such as thickness, homogeneity, interlayer packing, etc. Various deposition methods such as dipping, spraying, vacuum filtration, spinning, etc. have been applied to prepare multifunctional thin films and membranes.29−32 Among these methods, vacuum filtration is often used to prepare GO films and membranes due to its facile operation procedure.13 In this study, we have prepared several GO membranes using vacuum filtration. Although the obtained GO membranes look smooth and transparent from optical images (Figure 4a), scanning electron microscopy (SEM) images reveal wrinkles on the surface (Figure 4b), which is probably because of the sole vertical pulling force of vacuum filtration that does not allow ordered horizontal alignment of GO nanosheets along the surface of AAO substrates. Obviously, an optimized depositing method is needed for a better horizontal alignment of GO nanosheets onto the AAO surface to get smoother membranes with possibly better gas separation performance. Spin coating is another commonly used deposition method for the preparation of thin films and membranes.33 During this process, a solution containing materials to be deposited is spread out uniformly over the substrate under centrifugal force, resulting in thin and homogeneous films or membranes. This deposition method is also used in this study, hoping to get uniformly aligned GO nanosheets. We have found that a speed balance between deposition and solvent (water) evaporation is crucial to getting high-quality membranes. Faster deposition may lead to overflow of the solution, while faster evaporation will cause uneven distribution of GO nanosheets. Given the relatively high boiling point of water, the AAO substrate needs to be kept at 90 °C in order to allow a quick evaporation of water that can match up with the deposition speed. To be specific, 3 mL of aqueous solution containing exfoliated and purified GO nanosheets was deposited onto an AAO substrate, which was rotating at a low angular velocity and then gradually increased to 1000 rpm. The adhesive force on the AAO surface and the centrifugal force caused by rotation generated strong sheering force, resulting in a radial flow of the deposited GO solution along the AAO substrate. This process combined with the rapid evaporation of water on a hot AAO surface led to a uniformly covered GO membrane on the substrate. The whole spin coating process lasted for about 10 min. As has been expected, the surface of the GO membrane prepared through spin coating is much smoother than the one prepared by vacuum filtration, confirming the uniform alignment of GO nanosheets (Figure 4c). The morphology of the AAO substrate can be clearly seen in some areas (Figure 4d,e), indicating the ultrathin feature of the GO membrane. The thickness of the GO membrane is estimated to be ca. 20 nm (Figure 4f, Figure S4), which grants high gas fluxes for efficient gas separation. Gas Permeation Tests. The gas permeation tests were started by measuring an equal molar H2/CO2 mixed gas. We first conducted a controlled experiment using blank AAO
Figure 2. (a−d) AFM images of exfoliated GO nanosheets; (e, f) corresponding height profiles.
DLS is mainly applicable to spherical particles based on the Stokes−Einstein equation,27 it has been reported that DLS can also be used to determine the lateral size of nanosheets with a high aspect ratio following specific correlation. Coleman et al. demonstrated a correlation of particle size distribution obtained by DLS and the lateral size of nanosheets: aDLS = 5.9 × L0.66 (aDLS represents particle size distribution obtained by DLS, L represents lateral size of nanosheets).28 In this work, we have applied this correlation to the raw DLS data (Figure S3) in order to calculate the lateral size of GO nanosheets. As can be seen from the correlated data in Figure 3, the average lateral
Figure 3. Correlated particle size distribution data of GO nanosheets before and after purification. C
DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 4. (a) Optical image of prepared GO membrane on AAO support. (b) SEM image of GO membrane prepared by vacuum filtration (circled areas indicate wrinkles). (c−e) SEM images of GO membrane prepared by spin coating (circled areas indicate the morphology of AAO). (f) Crosssectional SEM image of GO membrane prepared by spin coating indicating a thickness of ca. 20 nm.
substrates. The obtained H2 permanence is 5.01 × 10−6 mol/ (m2·s·Pa) with a H2/CO2 separation factor of less than 5 that is within the expectation of Knudsen diffusion through 20 nm pores of pure AAO substrates.13 We prepared four GO membranes in parallel by vacuum filtration and tested their gas separation performance, all of which are quite close to each other (Table 1). The highest H2 permanence obtained in these GO membranes is 5.2 × 10−7 mol/(m2·s·Pa), which is 1 order of magnitude lower than that of blank AAO but much higher than most of the polymeric membranes confirming the expected high gas flux of ultrathin 2D GO membranes. However, the H2/CO2 separation factor is only 51, which is much lower than the reported value of 3400 obtained using GO membranes.13 For the membranes prepared by spin coating, the highest H2 permanence obtained is 35% lower than the previous case [3.4 × 10−7 mol/(m2·s·Pa) vs 5.2 × 10−7 mol/(m2·s·Pa)]. However, the H2/CO2 separation factor has been greatly increased by 370% (240 vs 51). The increased gas selectivity can be attributed to the uniformly aligned GO nanosheets by spinning coating, which minimizes the formation of wrinkles leading to smoother membranes as revealed by SEM images discussed previously. Such a high H2/CO2 selectivity indicates a dominating molecular sieving separation mechanism in which smaller H2 molecules (kinetic diameter 2.89 Å) can penetrate through the structural defects of GO nanosheets, while it is harder for larger CO2 molecules (kinetic diameter 3.3 Å) to go through (Figure 5a). However, we cannot completely rule out the possibility of gas passage through interlayer galleries, as has been suggested previously.15 In order to verify the molecular sieving separation mechanism of the GO membranes, we have tested the single gas permeance using six gases with various kinetic diameters. As shown in Figure 5b, the GO membranes allow faster permeance
Table 1. H2 Permeance and H2/CO2 Selectivity of Membranes Prepared in This Work and Several Reference Inorganic Membranes membrane
Temp.a
PH2b
α(H2/CO2)
ref.
MFI zeolite MFI zeolite DDR zeolite Zeolite composite Si600 SiC SiO2−TiO2 ZIF-7 ZIF [Zn2(bim)4] GO GO GO GO GO GO GO GO GO
500 500 25 200 200 200 600 220 25 20 25 25 25 25 25 25 25 25
3960 100 300 700 5000 89 2300 450 7701 1000 5070 5200 4900 5110 3300 3350 3400 3150
141 45.6 4 20 70 48 57 13.6 217 3400 48 51 44 48 232 218 240 238
34 35 36 37 38 39 40 41 42 13 c c c c d d d d
°C. b10−10 mol/(m2·s·Pa). cGO membranes prepared by vacuum filtration in this work. dGO membranes prepared by spin coating in this work.
a
of smaller gases (e.g., He and H2) but slower permeance of larger gases (e.g., CO2, N2, and CH4). For the largest gas SF6, the permeance has dropped to an almost undetectable level. The permeance order follows He > H2 ≫ CH4 > CO2 > N2 ≫ SF6, which is roughly the same order of their molecular size except CH4. Considering the similar sorption amount of various gases into GO membranes under testing conditions D
DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 5. (a) Scheme of molecular sieving H2/CO2 separation in GO membranes. (b) Permeance of single gases through GO membranes prepared by spin coating and vacuum filtration. (c) The effect of temperature on H2/CO2 permeance and H2/CO2 separation factor of the GO membrane prepared by spin coating. (d) H2 permeance versus H2/CO2 selectivity of GO membranes in this study along with several reference inorganic membranes listed in Table 1. Black line indicates the Robeson 2008 upper bound of pure polymeric membranes for H2/CO2 separation, assuming a membrane thicknesses of 100 nm. Red line indicates the 2010 upper bound of microporous inorganic membranes for H2/CO2 separation.
mol/(m2·s·Pa) and 1.0 × 10−8 mol/(m2·s·Pa), separately. The ideal H2/CO2 permselectivity (58) is also comparable with the separation factor (51) of the mixed H2/CO2 gas. The industrial H2/CO2 separation operations, such as those after water−gas shift reactions, are typically conducted at higher temperatures.4 Therefore, it is important to study the effect of temperature on the gas separation performance of membranes. In this study, we investigated the effect of temperature on the permeance and selectivity of H2/CO2 binary mixed gas using GO membranes prepared by spin coating. As shown in Figure 5c, the permeance of both H2 and CO2 increases with increasing temperature from 20 to 120 °C. At 120 °C, the H2 permeance has almost doubled compared with the permeance at 20 °C [6.7 × 10−7 mol/(m2·s·Pa) vs 3.4 × 10−7 mol/(m2·s· Pa)]. However, the permeance of CO2 increases at a higher speed, leading to a reduced H2/CO2 separation factor of 47 at 120 °C. This indicates that the permeance of CO2 is more sensitive toward temperature change, possibly because of the existence of interlayer galleries that prefer high-temperature gas permeation. Similar phenomena have been reported by other groups.13,42 The H2 permeance versus H2/CO2 selectivity of the GO membranes prepared in this study is plotted in Figure 5d along with several reference inorganic membranes and the 2008 Robeson upper bound for pure polymeric membranes and 2010
(Figure S5), the distinct difference of gas permeance indicates a molecular sieving separation mechanism. We have noticed that although CH4 has a kinetic diameter (3.758 Å) larger than that of CO2 (3.3 Å) and N2 (3.64 Å), its permeance through GO membranes is higher than those of the latter two. A similar abnormal behavior of CH4 has been observed in other GO membranes.13 We speculate it is probably because of the molecular shape effect. Compared to the rod-shaped CO2 and N2 molecules, the sphere CH4 molecules might be easier to permeate through nanopores of GO membranes due to less steric hindrance. However, further study is required to verify this hypothesis, which is beyond the scope of this study. The permeance of pure H2 in a GO membrane prepared by spin coating is 3.5 × 10−7 mol/(m2·s·Pa), which is comparable with the result obtained from H2/CO2 mixed gas tests [3.4 × 10−7 mol/(m2·s·Pa)]. The ideal H2/CO2 permselectivity calculated using pure gas data is 259, which is also close to the H2/CO2 separation factor of 240 from mixed gas tests. For the GO membrane prepared by vacuum filtration, a similar gas permeation trend is observed, in which He and H2 have the highest permeance while SF6 has the lowest one. Compared with the membrane prepared by spin coating, the gas permeance is higher in membranes by vacuum filtration, possibly because of the wrinkles and defects serving as fastpassing channels. The permeance of H2 and CO2 is 5.85 × 10−7 E
DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials upper bound for microporous inorganic membranes.41 The H2/ CO2 separation performance of the GO membranes in this study has exceeded both upper bounds. Although the H2/CO2 selectivity of the GO membranes prepared though spin coating in this study is still lower than the one reported previously,13 the H2 permeance reported herein has almost tripled the reported result (Table 1). Considering a balance between H2 permeance and H2/CO2 selectivity, the GO membranes prepared in this study are among the top of all the reported membranes for efficient H2/CO2 separations.
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vacuum conditions onto an AAO substrate to give the GO membrane, which was dried at 50 °C for 24 h before gas separation tests. Gas Permeation Tests. The prepared GO membrane was sealed in a home-built Wicke−Kallenbach permeation cell to measure its separation performance for either single gases or an equal molar H2/ CO2 mixed gas (Figure S6). In order to avoid the damage of the GO layer, the edge of the membrane disk was masked with an aluminum gasket coated with a silicone rubber pad, exposing only a 4-mmdiameter hole in the center of the membrane. The volumetric flow rate of gas (either single gas or mixed gas) was kept at 100 mL/min by mass flow controllers (Brooks Instrument). Argon was used as the sweep gas at a constant volumetric flow rate of 100 mL/min to eliminate concentration polarization in the permeate side. There was no pressure drop between the two sides of the membrane to prevent any distortion of the GO membrane. The separation factor is defined as the molar ratio of H2 to CO2 on the permeate side determined by gas chromatograph (Shimadzu GC-2014). The separation factor was calculated by the average of three measurements. In order to avoid the possibility of CO2 sorption in a silicone rubber pad, all of the tests were carried out after the establishment of a steady state (e.g., overnight equilibrium). Other Characterization. UV−vis absorption spectra were obtained with a Shimadzu UV-2600 spectrophotometer. AFM was done with a Bruker Dimension Icon atomic force microscope. Spin coating was carried out using a Laurell WS-400 spin coater. FE-SEM was conducted on a FEI Quanta 600 SEM (20 kV) equipped with an energy dispersive spectrometer (EDS, Oxford Instruments, 80 nm with 2 detector). Gas sorption isotherms were measured up to 1 bar using a Micromeritics ASAP 2020 surface area and pore size analyzer.
CONCLUSION
In conclusion, we have demonstrated a facile preparation of high-quality GO membranes that can be consistently repeated. Our preparation method integrates several procedures: (1) mild freeze−thaw exfoliation to get large GO nanosheets, (2) adjusting the solution pH value to purify GO nanosheets, and (3) spin coating to fabricate smooth GO membranes with uniformly aligned GO nanosheets. The GO membranes prepared in this study exhibit a great balance between H2 permeance [up to 3.4 × 10−7 mol/(m2·s·Pa)] and H2/CO2 selectivity (up to ∼240), which may find wide applications in H2 purification and precombustion CO2 capture. Single gas permeation tests and variable temperature tests indicate a dominating molecular sieving separation mechanism and possible gas diffusions through GO interlayer galleries. Given the facile preparation procedure and consistent gas separation results, this study has shed light on the large-scale production and application of GO membranes as promising gas separation materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04475. Additional information for the purification of GO nanosheets and apparatus scheme of gas permeation measurement (PDF)
EXPERIMENTAL SECTION
Freeze−Thaw Exfoliation of GO. Bulk GO powders (XF NANO, Nanjing) were gently ground using a mortar and pestle. The ground GO was dispersed in deionized water with a concentration of 0.05 wt % in a plastic tube (volume 50 mL). The tube was soaked in a liquid nitrogen bath and kept for 3 min to freeze the aqueous suspension, followed by soaking in a boiling water bath for 3 min to thaw the solid. After a total six freeze−thaw cycles, the GO solution was centrifuged at 3000 rpm for 5 min to remove large particles. The supernatant solution was withdrawn for further purification and characterization. Purification of Exfoliated GO. The pH value of aqueous GO solution obtained after freeze−thaw exfoliation was adjusted to 3.0 with 1 M HCl. After 30 min of standing, the upper half of the solution was decanted, and the bottom half was redissolved using deionized water and further treated similarly. The purification process was repeated five times to remove small GO particles from large GO nanosheets. Dynamic Light Scattering (DLS) Measurements. DLS measurements were performed with a Malvern Zetasizer Nano ZS. Aqueous solutions containing GO nanosheets before and after purification were tested in quartz cuvettes with a 10 mm path length. The volume of the sample was set at 10 mL. The equipment was operated in backscatter mode at an angle of 173°. The prepared samples were equilibrated at 25 °C for 2 min before each measurement. Fabrication of GO Membranes. GO membranes were prepared by spreading 3 mL of exfoliated GO solution onto a horizontally rotating AAO substrate (20 nm pores, diameter 47 mm). The rotating speed was gradually increased to 1000 rpm to have a uniform coverage of the GO membrane on AAO. During this process, the AAO substrate was kept at 90 °C to allow a quick evaporation of GO suspension. The prepared GO membranes were then dried at 50 °C for 24 h before a gas separation test. GO membranes were also prepared by vacuum filtration. The exfoliated GO solution (3 mL) was filtrated under
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National University of Singapore (CENGas R-261-508-001-646) and Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, Tier 2 R279-000-429-112).
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DOI: 10.1021/acs.chemmater.5b04475 Chem. Mater. XXXX, XXX, XXX−XXX
