Synthesis and characterization of rGO–TA and rGO–TH membranes

GO sheets were synthesized by exfoliation of graphite oxide made by the modified Hummers method29 using sonication. They are monolayers with a thickness of ~1 nm and lateral size mostly in the range 0.5–1.5 μm (Supplementary Fig. 2). Before membrane fabrication, the poly(ether sulfone) (PES) substrate was modified with polydopamine to enhance its adhesion with the GO-based membrane16. To fabricate rGO–TH (or rGO–TA) membranes, certain amount of GO and TH (or TA) with a weight ratio of 1:1 were first fully dispersed/dissolved in DI water by sonication and then stirred overnight at 70 °C for reaction. Different from the brown color of GO dispersion (Fig. 1a), the rGO–TH and rGO–TA dispersion show black color (Fig. 1b, c), indicating the reduction of GO sheets. The dispersion was then filtrated through the polydopamine modified PES membrane in a vacuum filtration cell. For comparison, GO membranes were also fabricated on modified PES substrates through vacuum filtration of GO dispersion. The thickness of membrane was controlled by the content of GO and TH (or TA) in the dispersion. Different from the tightly stacked layered structure of GO membranes (Fig. 1d), the rGO–TH and rGO–TA membranes show porous layered structure (Fig. 1e).

Fig. 1 Structure characterization of GO-based membranes. a–c Photos of GO dispersion (a), rGO–TA dispersion (b), and rGO–TH dispersion (c). d, e Typical cross-sectional SEM images of GO (d) and rGO–TH membranes (e). f–h C 1 s XPS spectra of GO (f), rGO–TH (g), and rGO–TA (h) membranes. i XRD patterns of GO, rGO–TA, and rGO–TH membranes. Scale bars: (d) 1 μm; (e) 5 μm Full size image

TA molecule (C 76 H 52 O 46 ) is very rich with hydroxyl groups (Supplementary Fig. 1a), and TH molecule (C 7 H 14 N 2 O 3 ) contains both amino and oxygen groups (Supplementary Fig. 1b). We used X-ray photoelectron spectroscopy (XPS) to identify the chemical composition of the membranes. As shown in Fig. 1f, the GO membrane shows similar XPS spectra with those of GO reported4,5,6, which contains four components that correspond to the carbon atoms in hydroxyl, epoxy, carbonyl, and carboxyl groups. It is important to note that although TA and TH are rich with oxygen functional groups, which, respectively, have an O/C atomic ratio of 0.61 and 0.43, the O/C atomic ratio decreases from 0.44 for pristine GO membrane to 0.40 for rGO–TA membrane and 0.22 for rGO–TH membrane. These results give strong evidence that the GO has been reduced, which is consistent with the color change mentioned above. In particular, the rGO–TH membrane shows typical C1s XPS characteristic of rGO (Fig. 1g), with greatly reduced peak intensity related to the oxygen functional groups. Because of the high O/C atomic ratio of TA molecules (0.61), the rGO–TA membranes show similar C1s XPS characteristic with GO (Fig. 1h).

We then used X-ray diffraction (XRD) to characterize the structure of the GO, rGO–TA and rGO–TH membranes. As shown in Fig. 1i, GO membranes show a single diffraction peak at 2θ of 11.6°, corresponding to an interlayer distance of 0.76 nm, attributed to the oxygen containing functional groups and the trapped water molecules between the stacked GO sheets. Interestingly, rGO–TA and rGO–TH membranes show two XRD peaks, which are different from the reported rGO membranes6,30. The small XRD peaks at around 26.4° are similar to those observed in graphite, which might be related to the narrowed interlayer distances of neighboring rGO sheets that are attracted each other by π–π interaction caused by the removal of oxygen functional groups during reduction. More importantly, rGO–TA and rGO–TH membranes also show strong diffraction peaks at 2θ of ~8.9° and 10.4°, respectively, corresponding to the interlayer distance of 0.99 nm and 0.85 nm. The large interlayer distance implies that the TA or TH molecules are inserted between the neighboring rGO sheets as spacer in most the regions of the rGO–TA and rGO–TH membranes.

Fourier transform infrared (FTIR) spectroscopy was used to identify the cross-linking between rGO sheets and TA/TH molecules (Supplementary Fig. 3). Compared to GO membranes, rGO–TA membranes show two strong new peaks at 1025 and 1718 cm−1, which correspond to the stretching vibration of the –C–O–C moiety and –C=O moiety in ester groups (–O–C=O), respectively31. These two peaks are also observed in TA molecules but show much weaker intensity. The increased peak intensities indicate that the residual carboxyl groups of rGO have covalently bonded with the hydroxyl groups of TA molecules, leading to more ester groups. Similarly, rGO–TH membranes show much stronger ester C–O stretch peak at 1389 cm−1. Moreover, the peak at 2357 cm−1, which is assigned to the –OH stretching of –COOH in TH molecules32, is absent in rGO–TH membranes. These confirm that the –COOH groups of TH molecules have been covalently bonded with the residual oxygen functional groups of rGO nanosheets. Therefore, there exists strong covalent interaction between rGO sheets and TA or TH molecules in the rGO–TA and rGO–TH membranes. In addition, there should also exist hydrogen bonding interactions between rGO sheets and TA/TH molecules and π–π interactions between rGO sheets and TA molecules.

The above characterizations suggest that the rGO–TH and rGO–TA membranes are a class of layered structure materials, which are composed of rGO membrane domains with both normal and enlarged interlayer distance. Due to the removal of oxygen functional groups, such membranes have greatly increased graphitic regions compared to GO membranes. First, the greatly increased pristine graphitic regions and enlarged interlayer distance in the expanded domains can facilitate rapid water permeance by nearly frictionless flow7. Second, the strong π–π attraction in the normal domains as well as the strong covalent bonding between rGO sheets and TA/TH molecules in the expanded domains are helpful for suppressing the movements of the rGO sheets. Third, the stacked rGO sheets together with the inserted TA/TH molecules can act as molecular sieves, blocking the solutes with hydrated radii larger than the size of nanochannels8. Therefore, it is expected that the rGO–TA and rGO–TH membranes have ultrahigh water permeance, good separation efficiency, and high stability in various aqueous solutions.

Stability of rGO–TA and rGO–TH membranes in aqueous solutions

We first measured the hydrophilicity of GO, rGO–TA, and rGO–TH membranes at 23 °C and 33% of humidity. As shown in Fig. 2a–c, although GO sheets are reduced by TA and TH molecules, the rGO–TA membrane (contact angle, ~26°) shows much better hydrophilicity than GO membrane (contact angle, ~54°), and the rGO–TH membrane is also hydrophilic with a contact angle of ~73°. As we know, the GO membranes easily swell in water because of the uptake of water, which significantly influences the separation performance and cycling stability. We then evaluated the water uptake of GO, rGO–TA (50 wt%) and rGO–TH (50 wt%) membranes according to equilibrium weight swelling ratio (ESR). As shown in Supplementary Table 1, the swelling of rGO–TA (ESR, ~1.3) and rGO–TH (ESR, ~1.8) membranes are much less than that of GO membrane (ESR, ~2.2), indicating that the movements of rGO sheets are suppressed in the rGO–TA and rGO–TH membranes.

Fig. 2 Hydrophilicity and stability of GO-based membranes. a–c Photos of a water droplet on GO (a), rGO–TA (b), and rGO–TH (c) membrane surface. d–l Stability in water (d–f), acidic solution (g–i), and basic solution (j–l). d, g, j GO membrane. e, h, k rGO–TA membrane. f, i, l rGO–TH membrane. The time staying in the aqueous solution is indicated in each photo Full size image

As mentioned above, the GO sheets tend to separate from each other in aqueous solutions due to the presence of oxygen functional groups, leading to damage and delamination of GO membranes. Therefore, long-term stability is essentially important for the solution-phase applications of GO membranes. We studied the stability of GO, rGO–TH, and rGO–TA membranes in different aqueous solutions. Similar to those reported previously, the GO membranes are readily disintegrated in water after 5 days (Fig. 2d). In contrast, the rGO–TA and rGO–TH membranes are very stable and remain their original structure even after 90 days (Fig. 2e, f). More importantly, the rGO–TA and rGO–TH membranes are also very stable in acidic (pH = 1.5, Fig. 2h, i) and basic (pH = 11, Fig. 2k, l) solutions, which is sharply different from the GO (Fig. 2g, j) and modified GO membranes reported so far11,25,26. As shown in Fig. 2g, j, the GO membranes are delaminated without shaking or stirring after 2 days in acidic and basic solutions, while no visible damage or delamination is observed for the rGO–TA and rGO–TH membranes even after 30 days (Fig. 2h, k, i, l).

Permeability and separation performance of rGO–TA and rGO–TH membranes

We then evaluated the permeability of our GO-based membranes with thicknesses ranging from 60 to 1500 nm (Fig. 3). The 100-nm-thick GO membrane shows a water permeance of 35 ± 5 L m−2 h−1 bar−1 (Fig. 3a), which is consistent with those reported with similar thickness14. This gives concrete validation for our measurement methodology. Surprisingly, the water permeance of rGO–TH membrane with a thickness of ~60 nm can reach 10,720 ± 30 L m−2 h−1 bar−1 (Fig. 3a). As shown in Supplementary Fig. 4, the water permeance greatly decreases with an exponential trend as the membrane thickness increases, which is very common for all the separation membranes due to the increase in mass transfer resistance. However, even when the rGO–TH membranes increase to 150 nm thickness, they still show a very high water permeance of 5000 L m−2 h−1 bar−1, which is more than 100 times higher than the 100-nm-thick GO membranes. More surprisingly, the rGO–TA membrane with a thickness of 150 nm shows a similar water permeance (10,191 ± 30 L m−2 h−1 bar−1) with the rGO–TH membrane with a thickness of 60 nm (Fig. 3a), indicating better permeability of rGO–TA membranes. This is because of the larger interlayer distance of expanded rGO membrane domains in rGO–TA membranes (~0.99 nm) than that of rGO–TH membranes (~0.85 nm) (Fig. 1i). When the thickness is decreased to 75 nm, the rGO–TA membranes show a higher water permeance (~11,600 ± 100 L m−2 h−1 bar−1) than the 60-nm-thick rGO–TH membranes.

Fig. 3 Permeability and separation performance of GO-based membranes. a DI water permeance of GO, rGO–TH, and rGO–TA membrane at a transmembrane pressure of 1.0 bar. b–d Permeance and rejection of organic dyes (50 µM) of GO (b), rGO–TA (c), and rGO–TH (d) membranes at a transmembrane pressure of 1.0 bar. GO membrane, 100 nm thick; rGO–TA membrane, 150 nm thick; rGO–TH membrane, 60 nm thick. e Permeance comparison of rGO–TH and rGO–TA membranes with the GO-based membranes reported in the literature14,15,16,17,18,19,20. Here, the permeance of rGO–TH and rGO–TA membranes is the maximum values that are obtained in the dye separation experiments Full size image

To further demonstrate the advantages of our rGO–TA and rGO–TH membranes over the GO membranes and reported membranes, we measured their separation performance through pressure-driven filtration using different organic dyes as model compounds, including RB, MLB, methyl blue (MB), and evans blue (EB). GO is known as a very strong adsorbent for many organic substances, including dyes16. In our studies, therefore, we have intentionally excluded the adsorption effect of GO sheets by stabilizing each rejection experiment prior to the collection of permeate, feed, and retentate samples for rejection analysis (Supplementary Fig. 5), which ensures correct assessment on the separation performance.

Figure 3b–d and Table 1 summarize the separation performances of our GO-based membranes for various dyes. It can be found that the rGO–TA and rGO–TH membranes show superior permeability and high separation efficiency. The permeance of rGO–TA and rGO–TH membranes is over 100 times higher than that of GO membranes for all the dyes. Because of different charge states, the rGO–TA and rGO–TH membranes show a bit lower rejection to negatively charged MB and EB but a bit higher rejection to positively charged RB and MLB than the GO membranes. For instance, the rGO–TH membranes show a rejection of 100% and permeance of 8526 ± 30 L m−2 h−1 bar−1 for MLB and a rejection of 71 ± 5% and permeance of 10,602 ± 30 L m−2 h−1 bar−1 for MB (Fig. 3d). Since the rGO–TA and rGO–TH membranes are negatively charged at neutral pH, positively charged molecules can be easily taken up by the membranes via electrostatic interactions. As a result, the nanochannels could be partially blocked, leading to a high rejection for cationic dyes17. As shown in Tables 1 and 2, it is worth pointing out that the rGO–TA and rGO–TH membranes exhibit a much better balance of permeability and rejection than the reported GO-based membranes14,15,16,17,18,19,20. In particular, the permeance is 10–1000 times higher than those of the previously reported GO-based membranes14,15,16,17,18,19,20 (Fig. 3e and Table 2). Supplementary Movies 1, 2 and Supplementary Fig. 6 clearly demonstrate the superior permeability and high separation efficiency of rGO–TH membranes. Such membranes should have a great potential for industrial water treatments.

Table 1 Separation performance of GO-based membranes for organic dyes Full size table

Table 2 Benchmarking of graphene-based membranes for organic dyes separation Full size table

We then investigated the influence of the membrane thickness and the content of TA/TH molecules on the separation performance of GO-based membranes. As shown in Supplementary Table 2, although the water permeance decreases, the dye rejection increases with an increase in membrane thickness. The 200-nm-thick rGO–TH membranes show over 10% increase in MB and EB rejection compared to the 60-nm-thick membranes and still a very high water permeance that is far better than the GO membranes and reported GO-based membranes. Even for the 800-nm-thick membranes, they still show a water permeance that is better than the most reported GO-based membranes but nearly 100% rejection for MB and EB. As shown in Supplementary Tables 3 and 4, as the weight ratio of TA or TH to GO sheets increases, the membranes exhibit increased rejection for organic dyes but decreased water permeance. This confirms that the TA and TH molecules not only serve as spacer to provide a large interlayer distance to facilitate water transport but also as molecular sieves to block the dyes. It is worth noting that the water permeance is still much higher than those reported even when the weight ratio reaches 4. For instance, the rGO–TH (1:4) membranes show water permeance of 2370 ± 30 L m−2 h−1 bar−1 and EB rejection of 98 ± 1%. These results suggest that the separation performance of the rGO–TA and rGO–TH membranes can be further improved by optimizing the thickness and the content of TA or TH of the membranes.

Synthesis, stability, and separation performance of rGO–GT membranes

As we know, GT extractive includes 30‒40% polyphenols (Catechins/TA) and 4‒6% amino acids (i.e., TH, glutamic acid, aspartic acid)33,34. To show the universality of our method, we fabricated rGO–GT membranes with GO and GT extractive. The rGO–GT membranes show similar structural features with the rGO–TA and rGO–TH membranes. As shown in Fig. 4a, b, the GO sheets are reduced by GT and the rGO–GT membrane shows an O/C atomic ratio of 0.44. Moreover, the membrane also has an enlarged interlayer distance of 0.87 nm (XRD peak at ~10.1°) (Fig. 4c), is hydrophilic (contact angle ~45°, Fig. 4d), and has a small swelling (ESR, 1.91 ± 0.21 in water) and very good stability in aqueous solutions with different pH values (Fig. 4e–g). As a result, the rGO–GT membranes also show much better separation performance than our GO membranes and those reported (Tables 1 and 2). For instance, the 500-nm-thick membranes show rejection of 100% for RB dye and high permeance of 1529 ± 10 L m−2 h−1 bar−1 (Fig. 4h). The big difference in XRD patterns between rGO–TA/TH and rGO–GT membranes might be due to the presence of other molecules beyond TA and TH in GT (over 50%) such as caffeine, theobromine, and theophylline33,34.