Graphene quantum dot membranes with tailorable pores for efficient gas separation

Domain: physics
Published: 2026-03-05
Editorial outcome: major_revision

Abstract
Membrane materials and fabrication methods often need to be co-developed to meet the specific permeance and selectivity requirements of different gas separation applications, which limits the adaptability of membrane technology. Here we propose a post-regulation strategy—adjusting the pore structures of a standard “primitive” membrane to meet various separation needs—that greatly simplifies membrane preparation and accelerates technological advancement. Graphene quantum dots (GQDs) are introduced as innovative building blocks: a continuous GQD membrane is constructed by tightly stacking GQDs, and then its pore structure is tuned via heat treatment and in-situ cross-linking with small amine molecules. Combining adjustable angstrom-scale pores with preferential CO2 adsorption, the resulting GQD membranes exhibit widely tunable CO2/N2 and CO2/CH4 separation performances. The CO2 permeance and separation factors surpass most reported membranes and can be tuned to exceed industrial targets for CO2 capture. By varying the heat treatment temperature, the separation scope of the membrane is further extended to challenging gas pairs such as C3H6/C3H8, demonstrating the high potential of this customizable post-regulation pore structure strategy.

Introduction
Membrane-based gas separation technology is highly efficient and energy-saving, making it a promising alternative to traditional separation methods, such as distillation and adsorption1–3. As the core of this technology, membrane materials have been extensively studied to achieve ultra-high permeance and selectivity. Advanced microporous materials, such as zeolites and metal-organic frameworks (MOFs) exhibits significant advantages in molecular sieving due to well-defined angstrom-scale pores4–8. For example, poly[Zn(benzimidazole)2]9–11 and zeolitic imidazolate framework-8 (ZIF-8)12–16 membranes have shown excellent H2/CO2 and CH4/C3H8 separation performance, respectively, by virtue pore sizes that ideally match the target molecules.

However, the molecular sieving capability of these porous materials, which is primary advantage, is also their fundamental limitation, as the separation efficacy is critically dependent on the intrinsic pore size. This necessitates searching for materials with pore sizes lying between the kinetic diameters of specific gas pairs and developing corresponding membrane preparation technologies, limiting the materials available for separation. Consequently, developing standardized primitive membranes with adjustable pore structures to meet specific permeance and selectivity requirements would greatly simplify membrane manufacturing process and facilitate commercial developments. This strategy circumvents the need to rebuild membranes from the ground up for each separation scenario and performance target. To make gas transport independent of the intrinsic pore structure of building blocks, the building block particles should be extremely small so that the bulk structure has minimal impact on the assembled membrane. In this context, zero-dimensional quantum dots are highly appealing for membrane construction.

Among them, emerging graphene quantum dots (GQDs, size <10 nm) possess stable graphitized carbon cores and abundant surface functional groups, enabling them to tightly self-assemble into three-dimensional networks through cross-linking chemical reactions17–20. Thus, GQDs are promising building blocks for assembling dense, defect-free membranes for gas separation. GQDsself-assembly is driven by their surface functionality, forming networks through covalent bonding, hydrogen bonding and electrostatic interactions among GQDs or between GQDs and cross-linkers21–23. For example, amino-functionalized GQDs are typically cross-linked with trimesoyl chloride (TMC) to engineer controlled nanochannels, successfully enhancing separation performance in nanofiltration membranes24,25. In addition, ultramicropores have been observed in microporous carbons derived from thermal annealing of GQDs with KOH activation, where the annealing induces “chemical welding” via C-O-C bonds formation26. Therefore, regulating the crosslinking process is a key process for achieving GQD membranes with tunable pore sizes. However, in existing studies, GQDs were mainly used as nanofillers to regulate the structure of the polymer continuous phase, and gas separation membranes utilizing GQDs as the main skeleton have rarely been reported.

Herein, we present a strategy to prepare a continuous GQD primitive membrane via thermal spraying, followed by pore-size regulation through molecular cross-linking using pyrolyzed polyethyleneimine (PEI). The thermally sprayed GQD primitive membrane is calcined under argon while simultaneously being cross-linked with PEI decomposition products to obtain a post-treated GQD (P-GQD) membrane. Benefiting from the pore sizes sieving effect of the regulated pores and the strong CO2 affinity of introduced surface groups, the P-GQD membrane exhibits CO2-selective permeation with permeance and selectivity tunable only by adjusting the cross-linking temperature. Moreover, at higher cross-linking temperatures, the P-GQD membrane can selectively separate C3H6/C3H8 mixtures. This small-molecule cross-linking approach provides a feasible path to prepare GQD membranes with flexible, post-fabrication regulation of pore structure, enabling customization for a variety of separation targets.

Results
Preparation and characterization of GQDs
Hydrophilic GQDs were synthesized by direct pyrolysis of citric acid (CA)27 at 210 °C for 0.5 h, as shown in Fig. 1a. Freeze-drying was used to avoid destroying the surface groups of GQDs during drying (Fig. 1b). A distinct Tyndall effect was observed in the aqueous GQDs sol (Fig. 1b right), indicating that the as-prepared GQDs are homogeneously dispersed in water. GQDs exhibit two key structural characteristics: (1) a stable graphitized carbon core, and (2) an abundance of oxygen-containing functional groups (-OH, -COOH, etc.) on the surface. In Fig. 1c, the high-resolution transmission electron microscopy (HR-TEM) image shows that the GQDs have irregular fragment morphologies with lateral dimensions less than 10 nm. Clear lattice fringes are observed with a lattice spacing of 0.32 nm, corresponding to the graphite (002) plane28. The image of atomic force microscopy (AFM) in Fig. 1d further indicates GQD thicknesses on the order of 3–5 nm, consistent with the HR-TEM results. Dynamic light scattering (DLS) spectrum of the aqueous GQDs sol displays a single dispersed peak of ~5 nm, confirming the uniformity of GQD nanoparticles (Supplementary Fig. 1).

The structure-induced absorption and emission spectral characteristics of GQDs were examined by UV-visible and photoluminescence (PL) spectroscopy. The UV-vis absorption spectrum (Supplementary Fig. 2) of GQDs displays broad bands in the near-UV region, which is the characteristic of surface functional groups and carbon core of GQDs29. Under 350 nm light excitation, the GQD solution shows a distinct fluorescence emission peak in the PL spectrum (Fig. 1e), attributed to the influence of surface functional groups30,31. The surface functional groups of GQDs were further identified by attenuated total reflectance-Fourier-transform infrared spectroscopy (ATR-FTIR), as shown in Fig. 1f. Peaks at 3035 cm−1 and 1406 cm−1 correspond to -OH stretching and bending vibration32. Peaks at 1703 cm−1 and 1177 cm−1 are attributed to C=O and C-O stretching vibrations of -COOH, respectively33. In addition, peaks at 2645 cm−1 and 905 cm−1 correspond to C-H and C=C vibration34. X-ray photoelectron spectroscopy (XPS) analysis of C 1s and O 1s spectra (Fig. 1g, h and Supplementary Fig. 3) further confirms the abundance of oxygen-containing groups on the GQD surfaces. The proportions of C=O, C-O and C=C groups are calculated to be 23.62%, 12.57% and 63.81%, respectively (Supplementary Table 1). These functional groups facilitate cross-linking between GQDs, making them attractive building blocks for separation membranes.

Preparation and characterization of P-GQD membranes cross-linked by PEI pyrolysis
As shown in Fig. 2a, a “primitive” GQD membrane was obtained by spraying the GQD sol onto a porous α-Al2O3 support. To prevent the small GQD nanoparticles from infiltrating into the macroporous support, which would impede gas transport and prevent formation of a surface layer, a thermal spraying method was used: the Al2O3 support was heated to 180 °C so that the solvent evaporated rapidly upon contact, thereby minimizing the penetration of GQDs and maintaining the substrate’s original permeability. At this optimized support temperature (180 °C, see Supplementary Fig. 4), GQDs remain on the surface. For the as-sprayed primitive GQD membrane, the proportion of C=O is almost constant, and the proportion of C-O increases, indicating that the carboxyl and hydroxyl groups are cross-linked (Supplementary Table 1). After spraying a total volume of 2 mL GQD sol, the particles stack into a thin continuous membrane, which can more effectively reduce gas transport resistance compared to thicker membranes (Supplementary Fig. 5). Moreover, the employed spray-coating is a scalable industrial process, which suggests its potential for the deposition of primitive GQD membranes onto large-area substrates with high reproducibility.

To enable selective gas transport across the membrane, either the pore size at angstrom level and/or preferential adsorption sites for specific gases need to be rationally regulated. The as-prepared GQD primitive membrane is initially gastight after thermal spraying at 180 °C, so a calcination step is required to create pores. We employed the pyrolysis of PEI to introduce abundant small amine molecules (e.g., diethylenetriamine and ethylenediamine)35 that can cross-link with the carboxyl groups on the GQD surface, thereby regulating the pore size of the membrane. Additionally, the introduction of amino groups, which have high CO2 affinity, should enhance CO2 selectivity36–38. Thermogravimetric analysis (TGA, Fig. 2b) shows that PEI begins to pyrolyze at 280 °C under inert atmosphere, releasing small amines that can cross-link with the GQDs framework. Above 330 °C, the weight loss of GQDs slows markedly, indicating a transition from rapid volatilization of unstable groups to slower removal of more stable groups. Based on these results, 350 °C was chosen as an initial trial temperature to ensure efficient cross-linking between PEI pyrolysis products and the GQD primitive membrane. The GQD membrane obtained via PEI pyrolysis cross-linking is hereafter designated as P-GQD.

The evolution of P-GQD membrane surface groups with calcination temperature was probed by quasi-in-situ infrared spectroscopy (Supplementary Fig. 6). Below 230 °C, neither GQDs nor PEI pyrolyze, and thus there is no significant change in functional groups. At 280 °C, the carboxyl and hydroxyl group of GQDs react with each other to form esters. After PEI starts to pyrolyze, the amino groups of small molecule amines react with the carboxyl groups of GQDs to form amide. Simultaneously, generation of -CN and enhanced adsorption of CO2 further indicate that small amines from PEI have chemically cross-linked onto the GQDs. As calcination temperature further increases, the content of O, N-containing functional groups has decreased, which is due to the continuous thermal decomposition of the cross-linked P-GQD membrane, causing the loss of partial elements.

The surface and cross-sectional morphologies of the P-GQD membrane were observed by a high-resolution SEM with energy-dispersive X-rays spectroscopy (EDS). As shown in Fig. 2c, a continuous, dense P-GQD membrane is formed on the alumina support, accompanied by a uniform distribution of N (Supplementary Fig. 7). This confirms that the GQD membrane was uniformly cross-linked by PEI pyrolysis products. To illustrate the role of PEI, we also prepared a control membrane (S-GQD) by calcining the single GQD membrane under Ar in the absence of PEI. In contrast to P-GQD membrane, the S-GQD membrane has a rougher surface showing only C and O from GQDs and Al from the Al2O3 support in EDS, with no N detected (Supplementary Fig. 8). Cross-sectional EDS mapping (Fig. 2d) clearly shows enrichment of C and N in the P-GQD membrane layer on the Al2O3 support surface and no significant penetration of GQDs into the support, indicating that the P-GQD membrane remains on top of the support. The uniformity of the prepared P-GQD membrane was further demonstrated by the enrichment of C and N elements on the support. The outline of alumina particles of the support is still visible (Fig. 2c), and Al signals appear in the XPS survey (Supplementary Fig. 9), suggesting the thickness of P-GQD membrane is ultrathin, which likely contributes to its high gas permeance.

The cross-linking by PEI pyrolysis was further verified by XPS analysis of nitrogen functionalities. The N 1s spectrum of P-GQD membrane (Supplementary Fig. 10) shows a peak attributable to C-N bonds in aromatic rings and secondary amine (C-NH-C) bonds, as shown in Supplementary Fig. 1139. Changes in the C 1s spectra (Fig. 2e and Supplementary Table 1) also reflect the introduction of PEI pyrolysis products during the preparation of the P-GQD membrane. The C 1s peaks at 284.8, 286.0, and 288.8 eV correspond to C=C, C-O/C-N, and C=O bonds, respectively40. After calcination, the intensity of the C=O peak decreases in both S-GQD and P-GQD membrane, indicating consumption of carboxyl groups via cross-linking. In the S-GQD (no PEI) membrane, a decrease in the C-O peak is also observed, due to partial volatilization of -COOH/-OH groups upon heating. In the P-GQD membrane, however, the 286.0 eV peak (C-O/C-N) is enhanced, reflecting the formation of new C-N bonds from cross-linking (in addition to residual C-O).

Raman spectroscopy (Fig. 2f) was used to analyze the carbon microstructure of the P-GQD membrane before and after calcination. Two characteristic peaks appear at ~1370 cm−1 (D-band, disordered carbon) and ~1605 cm−1 (G-band, graphitic carbon)41,42. The as-sprayed primitive GQD membrane exhibits a strong fluorescence background, making its D/G bands difficult to detect43; a faint G-band is visible, consistent with the high graphitization of GQDs. In both S-GQD and P-GQD membranes, pronounced D and G bands are observed. Notably, the P-GQD membrane shows a relatively stronger D-band (weaker G-band) compared to S-GQD membrane, indicating that the incorporation of PEI-derived alkyl chains increases the disorder in the carbon framework. XRD and HRTEM were also used to characterize the structural features of the P-GQD membrane, showing that the graphitic structure of GQD particles is preserved within the P-GQD membrane, and that interparticle stacking builds transport pathways (Supplementary Figs. 12 and 13).

Water contact angle measurements further confirmed the effect of PEI cross-linking on P-GQD membrane (Fig. 2g). The as-sprayed GQD primitive membrane is highly hydrophilic (low contact angle) due to abundant surface carboxyl and hydroxyl groups. After heat treatment without PEI (S-GQD), some of these groups are lost, resulting in reduced hydrophilicity of S-GQD membrane. In contrast, the P-GQD membrane surface becomes hydrophobic, owing to the hydrophobic alkyl chains introduced by cross-linking and the substantial reduction of carboxyl and hydroxyl groups. This enhanced hydrophobicity should help prevent moisture from disrupting the cross-linked network, thereby improving P-GQD membrane stability. Consistently, zeta potential measurements (Supplementary Fig. 14) show that pristine GQDs carry a significant negative charge (from deprotonated –COOH), while the zeta potential of P-GQD membrane is much less negative due to the neutralization of carboxylates by the cross-linked amines44.

Gas separation performance of P-GQD membranes
The gas separation performance of P-GQD membranes was evaluated by the Wicke-Kallenbach method for equimolar binary gas mixtures. The control S-GQD membrane shows low CO2/N2 separation factor (Supplementary Fig. 15). In contrast, P-GQD membranes exhibit greatly enhanced CO2 separation due to the PEI-derived cross-linked structure. As shown in Supplementary Fig. 16, for a P-GQD membrane prepared using 2 mL GQD sol and 6 mg PEI, the CO2 permeance increases with higher calcination temperature, while the mixed-gas CO2/N2 and CO2/CH4 separation factor decrease. This trend is attributed to more C, N, O being volatilized at higher temperatures, enlarging pore aperture and reducing steric hindrance to gas transport. Overall, the P-GQD membrane calcined at 350 °C exhibits the best performance, with a CO2 permeance of ~1504 GPU and a CO2/N2 separation factor of ~43, along with a CO2/CH4 separation factor of ~51.

To further probe permeation behavior, we varied the feed pressure in separation tests. As shown in Fig. 3a, b, both the CO2 permeance and separation factors of the P-GQD membrane decline as feed pressure increases, while the permeance of CH4 increases. This phenomenon is likely because the elevated pressure increases the CH4 loading in the membrane, thereby promoting CH4 permeation and hindering CO2 diffusion45. Even at 0.6 MPa feed pressure, the CO2/N2 and CO2/CH4 separation factors remain above 20, demonstrating robust high-pressure separation performance. Moreover, when the feed pressure is returned to 0.1 MPa, the CO2 permeance and separation factors recover to their initial high values, indicating that the membrane structure is not harmed by the pressure cycling.

We also evaluated the P-GQD membrane under different feed compositions. As shown in Supplementary Fig. 17, altering the CO2 content in the CO2/N2 or CO2/CH4 feed has minimal effect on CO2 permeance and separation factors, demonstrating that the P-GQD membrane offers high separation efficiency in separation scenarios with different CO2 contents. Increasing the test temperature accelerates gas diffusion46 and thus raises the CO2 permeance (Supplementary Fig. 18). However, the CO2/N2 and CO2/CH4 separation factors decrease at higher temperature because the adsorption affinity for CO2 is reduced. The performance of the P-GQD membrane was also evaluated under humid conditions. When exposed to humid feed gas at 60 °C, the P-GQD membrane maintains moderate permeance and selectivity, with performance recovery observed after returning to dry gas at room temperature (Supplementary Fig. 19).

The P-GQD membrane shows excellent stability over time. Over a 25 h continuous test with an equimolar CO2/N2 mixture, the CO2 permeance remains nearly constant and the CO2/N2 separation factor stays around 50 (Fig. 3c). Over longer-term tests totaling 100 h for CO2/CH4 and CO2/N2 separation with periodic feed pressure cycling, the CO2 permeance shows only a slight decline while the separation factor slightly increases (Fig. 3d and Supplementary Fig. 20). Under actual industrial conditions involving fouling and various gas impurities, the durability of P-GQD membrane needs to be further investigated. Notably, CO2/N2 separation performance of P-GQD membrane meets the industrial requirements for post-combustion CO2 capture (CO2 permeance ≥ 1000 GPU and CO2/N2 selectivity ≥ 20)47, and its CO2/CH4 separation performance far exceeds that of commercial natural gas separation membranes (typically CO2 permeance > 100 GPU, CO2/CH4 selectivity > 30)37, as illustrated in Fig. 3e, f. Importantly, by simply adjusting the calcination temperature, the permeance and selectivity of P-GQD membrane can be tuned over a wider range for both CO2/N2 and CO2/CH4 separation. For example, increasing the calcination temperature from 320 °C to 360 °C adjust the CO2 permeance from 120 to 1975 GPU, while the corresponding CO2/CH4 separation factor decreases from 138 to 28. This flexibility enables convenient customization of membrane performance for different separation scenarios.

Gas separation mechanism of P-GQD membrane
The P-GQD membrane has high CO2 permeability and selectivity, mainly originating from two key effects: preferential adsorption of CO2 and size-sieving by the regulated ultra-micropores (Fig. 4a). Since PEI contains abundant amino groups, the cross-linking of PEI pyrolysis products effectively enhances the CO2 affinity of the membrane, leading to selective CO2 adsorption and transport. Accordingly, compared with the GQD primitive membrane and the S-GQD membrane, the P-GQD membrane shows a distinct FTIR absorption band for adsorbed CO2 at 2340 and 2360 cm−1 spectrum (Fig. 4b).

To further elucidate the separation mechanism, gas adsorption experiments were conducted for single-component H2, CO2, N2 and CH4. As shown in Fig. 4c, the equilibrium adsorption capacity on P-GQD powder follows the order CO2 >> CH4 > N2 > H2, reflecting a strong preferential adsorption of CO2 by groups containing N. Temperature-dependent adsorption measurements (Supplementary Fig. 21) further verify the high CO2 affinity of P-GQDs: the zero-coverage adsorption enthalpy for CO2 is −28.14 kJ mol−1, much higher than that for CH4 (−16.66 kJ mol−1) or N2 (−11.86 kJ mol−1). In comparison, for S-GQDs calcined at 350 °C alone, the zero-coverage adsorption enthalpy of CO2 was measured to be −25.97 kJ mol−1, which is lower than that of P-GQDs (Supplementary Fig. 22). By fitting the 298 K isotherms (Supplementary Fig. 23) using ideal adsorbed solution theory (IAST), we estimated the adsorption selectivities to be ~14.6 and ~4.7 for equimolar binary CO2/N2 and CO2/CH4, respectively.

Membrane separation performance is also influenced by the degree of cross-linking. As shown in Fig. 4d, increasing the PEI dosage increases the CO2/N2 and CO2/CH4 separation factors but lowers the CO2 permeance because the excessive cross-linking reduces pore volume. Conversely, a too-low PEI amount (3 mg) is insufficient to form an effective cross-linked network, resulting in low separation factors (Supplementary Table 2). Thus, both excessive and insufficient cross-linking are detrimental to the separation performance of the P-GQD membrane.

The pore size distribution of P-GQD was examined using CO2 adsorption isotherm at 273 K (Supplementary Fig. 21a), indicating a bimodal microporous structure, with pore size around 0.35 nm lies between the kinetic diameters of CO2 (0.33 nm) and N2 (0.364 nm) (Fig. 4e). Single-gas permeation tests yield ideal selectivities of ~29 for CO2/N2 and ~44 for CO2/CH4 (Fig. 4f, g), far exceeding the corresponding Knudsen diffusion selectivities (0.8 for CO2/N2, 0.6 for CO2/CH4) and also much higher than the IAST adsorption selectivities. This indicates that molecular sieving (exclusion of larger molecules by the ~0.35 nm pores) plays an indispensable role in the CO2 separation process.

The temperature dependence of permeance provides further insight. As test temperature rises, permeances of all gases increase due to accelerated diffusion. However, the increase in CO2 permeance is significantly smaller than that of CH4 or N2, because higher temperature weakens CO2 adsorption affinity (Supplementary Fig. 18). From Arrhenius plots of permeance vs 1/T, we calculate permeation activation energies of 7.65 kJ mol−1 for CO2, 22.36 kJ mol−1 for N2, and 23.61 kJ mol−1 for CH4 (Supplementary Fig. 24). The notably lower activation energy for CO2 indicates that the membrane facilitates CO2 transport relative to the other gases. Finally, we observed that using a larger amount of GQD sol (forming a thicker membrane) decreases CO2 permeance but increases separation factors (Supplementary Fig. 25), consistent with increased diffusion path length and additional selective adsorption sites. All these results demonstrate that the P-GQD membrane achieves CO2 separation through a combination of strong CO2 adsorption and size-sieving effects. It is worth noting that GQD membranes cross-linked with other small amine (e.g., diethylenetriamine) also show moderate CO2 separation performance (Supplementary Fig. 26), suggesting the generality of this small-molecule post-cross-linking strategy. However, their efficacy is somewhat lower than that of PEI-cross-linking due to the weight loss mismatch and the inability of a single small molecule to precisely match the subtle variations in the GQDs stacking pores.

C3H6/C3H8 separation performance of regulated P-GQD membrane
Based on the pore formation mechanism and the CO2 separation results, varying the calcination temperature modifies the degree of crosslinking and volatilization of some light elements, thereby altering the pore size of the P-GQD membrane (Supplementary Fig. 27). P-GQDs exhibit a bimodal ultra-microporous structure with sizes of 0.33–0.40 nm and 0.43–0.70 nm (Supplementary Fig. 28). The total micropore volume increases with calcination temperature due to enhanced release of volatile species. A fundamental trade-off exists: >0.4 nm pores become more, whereas <0.4 nm pores required for size-sieving of CO2 is maximized at intermediate temperature (for instance 350 °C) and diminishes at 380 °C due to pore widening. Therefore, we expected that increasing the calcination temperature (and thus pore size) would enable separation of larger gas molecules.

To test this, we carried out single-component permeation experiments on P-GQD membranes calcined at various temperatures. As shown in Fig. 5a, b, at a relatively low calcination temperature of 320 °C, the small pore size of the membrane significantly blocks the permeation of gases larger than CO2. As the calcination temperature increases, the permeance of all gases rises and the CO2/N2 selectivity gradually drops, but a clear cut-off between C3H6 and C3H8 emerges. In an equimolar C3H6/C3H8 mixture test, the P-GQD membrane calcined at 380 °C achieves a C3H6 permeance of 117 GPU with a C3H6/C3H8 selectivity of 12. This selectivity is far higher than the IAST-predicted adsorption selectivity (1.98, Supplementary Fig. 29), underscoring the dominant role of molecular sieving due to pore widening. The C3H6/C3H8 selectivity remains above 10 even as the feed pressure increases to 0.35 MPa (Fig. 5c). These results demonstrate that the P-GQD membrane has potential application in C3H6/C3H8 separation. Unlike previous studies, small amounts of GQDs doping only modulate the properties of mixed matrix membranes, here GQDs were employed as building blocks to assemble into dense membranes. More significantly, the membrane preparation strategy we developed allows straightforward tuning of pore size to match different separation targets by simply adjusting the calcination temperature and the extent of cross-linking. This is a convenient “post-regulation” approach compared to synthesizing entirely new membranes for each application. By formulating appropriate post-treatment conditions for different separation scenarios, this strategy could greatly accelerate the development of high-performance membranes for a broad range of gas separation.

Methods
Chemicals and Materials
CA (CA, 99%) was purchased from Macklin. Polyethyleneimine (PEI, M.W. 10000, 99%) was obtained from Aladdin. α-Al2O3 supports (top-layer pore size ~70 nm) were provided by Foshan Yirun Jingtao New material Co., Ltd.

Synthesis of GQDs
GQDs were synthesized through pyrolysis of CA. Specifically, 1.5 g of CA powder was placed in a beaker and heated at 210 °C for 30 min in an oven. After pyrolysis, the resulting brown-red solid was dissolved in 20 mL deionized water with stirring, then filtered with a 0.22 μm membrane to obtain a GQD sol. The concentration of GQDs aqueous sol is approximately 10 mg mL−1. The GQD sol was freeze-dried to yield GQD powder for characterization.

Preparation of GQD primitive membrane by thermal spraying
A porous α-Al2O3 support (top layer pore size of 70 nm, diameter of 18 mm and thickness of 1 mm) was preheated to 180 °C on a hot plate. A GQD aqueous sol was then spray-coated onto the hot support using a handheld spray gun (0.3 mm nozzle) with compressed air (flow of 7–7.5 mL/min). The spray nozzle was held vertically ~15 cm above the Al2O3 support, and the GQD sol was delivered at ~0.15 mL/min. Upon contact with the heated support, the solvent water droplets evaporate quickly, leaving GQD nanoparticles on the support surface to form a continuous GQD primitive membrane. A total of 2 mL of GQD sol was sprayed. The actual amount of GQDs loaded on the support was determined by weighing the substrate before and after spraying (Supplementary Table 5). After coating, the GQD primitive membrane was further dried in an oven at 80 °C for 10 h.

Preparation of P-GQD membrane by PEI pyrolysis cross-linking
The dried GQD primitive membrane was placed in a tubular furnace together with a specified amount of polyethyleneimine (PEI) for heat treatment under flowing Ar (100 mL min−1). The PEI was positioned upstream of the GQD primitive membrane so that its decomposition vapors could contact and cross-link with the surface functional groups of GQD membrane. The system was heated to the target temperature at 1 °C min−1 and maintained for 10 h. After cooling to room temperature under Ar, the cross-linked P-GQD membrane was obtained with the regulated pore structure. For comparison, an S-GQD membrane was prepared by heat treating the sprayed GQD primitive membrane under the same conditions but without any PEI present.

Gas permeation tests
Each membrane was mounted in a custom stainless-steel permeation cell and sealed with silicone O-rings. The diameter of the tested P-GQD membrane is 10 mm. Gas permeation was measured by the Wicke-Kallenbach method at room temperature. For single-gas tests, H2, CO2, N2, CH4, C3H6, or C3H8 was fed to the upstream side at a flow rate of 100 mL/min. For mixed-gas tests, equimolar binary CO2/N2 or CO2/CH4 feed was used, and helium (He) at 50 mL/min was used as the sweep gas on the permeate side. The compositions of permeate gases were analyzed online by a gas chromatograph (Agilent 7890B) equipped with a high-sensitivity thermal conductivity detector. Gas permeance was calculated as:

Pi = Ji / (S * Δpi)

where Pi is the permeance of component i (mol m−2 s−1 Pa−1), Ji is the molar flow rate of component i in the permeate (mol s−1), S is the effective membrane area (m2), and Δpi is the transmembrane pressure drop of component i. Permeance is often reported in gas permeation units, GPU, where 1 GPU = 3.349 × 10−10 mol m−2 s−1 Pa−1.

The ideal selectivity for gases i and j was calculated as:

αij = Pi / Pj

where Pi and Pj are single gas permeance of component i and j, respectively.

For gas mixtures, the separation factor was determined by:

SFij = (yi / yj) / (xi / xj)

where x and y are the molar fractions of components i and j in the feed and permeate, respectively.

The temperature dependence of permeance was analyzed using the Arrhenius equation:

ln P = −Ep / (R * T) + C

where P is the gas permeance, Ep is the activation energy of gas permeation, R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature (K) and C is constant. By plotting ln P is versus 1/T, the activation energy Ep can be obtained from the slope.

Characterization of GQDs
The morphology and size of GQDs were examined by transmission electron microscopy (TEM, JEM-2100, JEOL) and AFM (AFM, Nanowizard, JPK). Surface functional groups were characterized by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR, Nicolet iS50) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250 Xi+). Optical properties were measured by Ultraviolet-visible spectroscopy (UV-vis, UV-1900) and photoluminescence spectroscopy (PL, PTI QM400). Thermogravimetric analysis (TGA) of GQDs and PEI was performed using a Netzsch STA 449 F5 thermal analyzer and a PerkinElmer Diamond TG/DTA, respectively, at a heating rate of 10 °C min−1 from 50 °C to 800 °C under N2 flow rate of 20 mL min−1.

Characterizations of GQD membranes
Surface and cross-sectional morphologies of the sprayed GQD primitive membrane and P-GQD membrane were observed by scanning electron microscopy (SEM, JSM-7900F, JEOL) with energy-dispersive X-rays spectrometer (EDS) for elemental distribution mapping. Fourier-transform infrared spectroscopy (FT-IR, TENSOR27, Bruker) was used to monitor the evolution of membrane surface groups under different heat-treatment temperatures. Elemental composition of the membranes was analyzed by XPS (Thermo Fisher Escalab 250 Xi+). Carbonaceous microstructures of membranes were examined by Raman spectroscopy (HNG-IS Raman, 355 nm excitation). Water contact angles were measured using a KRÜSS DSA100 contact angle goniometer for evaluating hydrophilicity and hydrophobicity of membranes. Zeta potential measurements of membrane samples were carried out using a Zetasizer Nano instrument (Malvern Co.).

Gas adsorption experiments
P-GQD powder samples for adsorption measurements were prepared by the same PEI pyrolysis cross-linking process as the membrane. Before measurement, samples were degassed at 200 °C for 3 h to remove residual volatiles. Gas adsorption isotherms for H2, CO2, N2, and CH4 were collected at 298 K and 273 K using volumetric adsorption analyzers (Micromeritics ASAP 2020 Plus and Quantachrome Autosorb iQ). The 298 K adsorption data for CO2, N2, and CH4 were fitted with a single-site Langmuir-Freundlich model:

q = Q * (bp)n / (1 + (bp)n)

where q is the amount of adsorbed (mmol g−1) at pressure p, Q is the saturation adsorption capacity (mmol g−1), b is the affinity constant (kPa−1), and n is the heterogeneity index.

The ideal adsorption solution theory (IAST) was used to predict adsorption selectivity for binary mixtures at equilibrium. The IAST selectivity (Sads) for component 1 over component 2 is given by:

Sads = (q1 / q2) / (p1 / p2)

where q1 and q2 are adsorbed amounts of component 1 and 2, and p1 and p2 are the corresponding mole fractions in the gas phase.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work presents a post-regulation strategy for fabricating gas separation membranes by constructing a continuous primitive membrane from GQDs, followed by pore structure tuning via thermal treatment and in-situ cross-linking with amine molecules. I suggest a major revision for the manuscript, and some concerns in this work still need to be addressed：\n1.There is already a substantial body of literature employing quantum dots as separation layers. The authors should more explicitly highlight the unique structural and performance advantages of their work compared to existing studies.\n2.Regarding Fig. 1, the authors did not provide the size distribution of the synthesized GQDs. How was the uniformity of the quantum dots ensured?\n3.The thickness measured by AFM (3–5 nm) suggests that the GQDs are multi-layered. How did the authors account for the potential influence of the number of layers on the membrane structure and separation performance?\n4.The alumina substrate was heated to 180 °C. How did the authors confirm that this temperature does not adversely affect the structure of the GQDs?\n5.The authors mention the regulation of angstrom-scale pores, but direct characterization and explanation of this critical feature are lacking in the article.\n6.The cross-linking reaction between pyrolyzed PEI products and GQDs is a crucial step in modulating the membrane structure, yet the description of the reaction mechanism remains somewhat vague.\n7.High-temperature calcination may alter the stacking mode, interlayer spacing, or defect states of the GQDs—physical structural aspects that were not analyzed (e.g., via XRD, BET, or HRTEM). It is recommended to supplement the physical structural characterization of the membranes after calcination to establish a more comprehensive structure–performance relationship.\n8.The pore size shown in Fig. 4e is presented as a single value, but given the inherent size distribution of the GQDs, the resulting pores would also be expected to exhibit a distribution. Could the authors clarify this?\n9.While using mixed amine compounds from PEI pyrolysis for cross-linking enables pore size regulation, the precise ratios and reactivity of these components are challenging to control. Could the authors elaborate on the rationale for choosing PEI pyrolysis over cross-linking with a single small molecule amine?", "reviewer_2": "In this study, the authors have successfully developed a graphene quantum dot (GQD) membrane with high efficiency for CO2 separation. By utilizing GQDs as novel building blocks, they constructed the fundamental membrane architecture, followed by precise modulation of the pore structure through heat treatment and in situ cross-linking with small-molecule amines. The resulting membrane demonstrates exceptional CO2 separation performance, which exceeds the industrial separation target. Notably, the proposed strategy for pore structure customization not only shows promising potential for C3H6/C3H8 separation but also provides valuable insights for the rational design of membrane structures tailored to diverse separation systems. Given the significance and innovation of this work, I recommend the manuscript for publication in Nature Communications after addressing the following issues(minor revision):\n1. A consideration is whether graphene quantum dots (GQDs) have the ability to penetrate into the substrate’s internal structure, such as pores or surface defects, and if so, how this penetration might alter the substrate’s intrinsic properties.\n2. It is strongly recommended that the loading amount of graphene quantum dots onto the support during the spraying process be quantitatively determined. This information is essential for establishing a clear correlation between GQD content and membrane performance, as well as for ensuring reproducibility of the fabrication process.\n3. Regarding the formation of a continuous GQD membrane on the Al2O3 support, further clarification is needed: does the spraying process alone yield a continuous, defect-free membrane structure, or is subsequent cross-linking with PEI pyrolysis products necessary to eliminate pinholes, enhance interparticle adhesion, and achieve the required continuity for effective separation?\n4. Generally, the support and membrane area will affect the separation performance of the membrane. Please provide the specifications of the alumina support used and the GQD membrane area tested.\n5. The observed trend that an increase in feed pressure leads to a decrease in CO2 permeance but an increase in CH4 permeance, requires a thorough explanation.\n6. It is necessary to carefully check the reference format of the manuscript and revise it to meet the requirements.\n7. I am wondering if the authors could put a schematic picture in the main text to illustrate how to precisely control the pore size of the GQD membranes. This is the most amazing part of this work. Moreover, what is the reproducibility of the membranes?", "reviewer_3": "This work required major revision before it is accepted for publications. Following are my comments:\nComment 1. The introduction is comprehensive and provides a strong basis for using GQDs in membrane-based separations. However, it is quite dense with information and could be streamlined. For example, you may consider separating background on conventional materials (zeolites, MOFs, etc.) from the motivation for GQDs to improve readability and emphasize the novelty of your approach.\nComment 2. Mention the concentration of GQDs utilized for gas separation applications. In the manuscript, it is mentioned that “a primitive GQD membrane was obtained by spraying the GQD sol onto a porous α-Al2O3 support………by spraying a total volume of 2 mL GQD sol, the particles stack into a continuous membrane”. Also, in Figure S4 of the SI, SEM images of the surface morphology of the sprayed GQD membrane with different volumes of GQD sol have been presented. Why is 2 mL spraying considered as optimum volume for further applications?\nComment 3. PEI pyrolysis introduces amino groups for CO2 affinity and pore regulation; the discussion could benefit from quantitative evidence of interaction. For instance, DFT calculations, binding energy analysis would make the mechanistic argument more convincing.\nComment 4. In Figure S1 of the SI, the UV-vis absorption peak of GQDs is not distinct, though it shows a broad band.\nComment 5. The membrane is prepared via thermal spraying + PEI cross-linking. Can this process be scaled beyond small lab samples? Is the method compatible with larger-area substrates? A brief comment on scalability and fabrication reproducibility would strengthen the practical relevance.\nComment 6. The XRD patterns of the synthesized materials are not given in the manuscript, which are important for determining the crystallinity of the membranes.\nComment 7. Provide the Robeson upper bound curve, indicating the synthesized membranes’ performance.\nComment 8. Stability tests (100 h, pressure cycling) are promising, but many industrial gas separation processes involve humidity or mixed contaminants (e.g., SO2, H2O vapor). Since GQDs are inherently hydrophilic/hydrophobic depending on cross-linking, more discussion of long-term stability in humid/mixed-gas environments is needed.\nComment 9. The propylene/propane results are impressive, but the mechanistic explanation is underdeveloped. Why exactly does the cut-off emerge at 380 °C? Is it purely pore widening, or do changes in surface chemistry (loss of functional groups) also contribute?\nComment 10. You demonstrate strong stability over 100 h, which is impressive. However, industrial applications often require much longer-term stability (weeks to months). Even if such tests are not feasible now, discussing potential long-term challenges (e.g., fouling, plasticization, or hydrothermal stability) would make the conclusions more compelling for application-focused readers."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1:\nThis work presents a post-regulation strategy for fabricating gas separation membranes\nby constructing a continuous primitive membrane from GQDs, followed by pore\nstructure tuning via thermal treatment and in-situ cross-linking with amine molecules.\nI suggest a major revision for the manuscript, and some concerns in this work still need\nto be addressed:\nResponse: We greatly appreciate reviewer’s insightful comments on our manuscript.\nWe have carefully revised the manuscript based on these comments, as detailed below.\n1. There is already a substantial body of literature employing quantum dots as\nseparation layers. The authors should more explicitly highlight the unique structural\nand performance advantages of their work compared to existing studies.\nResponse: Thank you for your kind suggestion. Indeed, in existing studies, graphene\nquantum dots (GQDs) were mainly used as nanofillers to modify the structure of\npolymer continuous phase, with very small amounts of GQDs incorporated. In this\nwork, GQDs were used as building blocks to assemble into dense membranes, with the\npore structure modulated by heat treatment and in-situ cross-linking with small\nmolecule amines. The synthesized P-GQD membrane achieved excellent CO\n2\nseparation performance, exceeding industrial separation targets, and the performance\ncan be customized to meet specific separation requirements. In addition, by increasing\nthe cross-linking temperature, the P-GQD membrane can also be used for C H /C H\n3 6 3 8\nseparation.\nFollowing the reviewer’s comment, the following two sentences were added on Page 4\nand 17 of the revised MS to address the advantages of our work, and the revision was\nmarked with yellow background.\n“However, in existing studies, GQDs were mainly used as nanofillers to regulate the\nstructure of the polymer continuous phase, and gas separation membranes utilizing\nGQDs as the main skeleton have rarely been reported.”\n“Unlike previous studies, small amounts of GQDs doping only modulate the properties\nof mixed matrix membranes, here GQDs were employed as building blocks to assemble\ninto dense membranes.”\n2. Regarding Fig. 1, the authors did not provide the size distribution of the synthesized\nGQDs. How was the uniformity of the quantum dots ensured?\nResponse: We are very grateful to the reviewer for the valuable suggestion. The size\ndistribution of the synthesized GQDs was analyzed by dynamic light scattering (DLS).\nAs shown in Fig. R1, the spectrum of the aqueous GQDs sol displays a single dispersed\npeak of ~ 5 nm, confirming the uniformity of GQD nanoparticles.\nFig. R1 Dynamic light scattering spectrum of the GQDs aqueous sol.\nFollowing the reviewer’s comment, the above Fig. R1 was added as Supplementary Fig.\n1 in the revised SI, and the following sentence was added on Page 5 of the revised MS\nto support the revision of the relevant issue.\n“Dynamic light scattering (DLS) spectrum of the aqueous GQDs sol displays a single\ndispersed peak of ~ 5 nm, confirming the uniformity of GQD nanoparticles\n(Supplementary Fig. 1).”\n3. The thickness measured by AFM (3–5 nm) suggests that the GQDs are multi-layered.\nHow did the authors account for the potential influence of the number of layers on the\nmembrane structure and separation performance?\nResponse: We thank the reviewer for this comment. Firstly, the main structure of the\nP-GQD membrane is mainly a three-dimensional network formed by the accumulation\nof GQD nanoparticles, so the accumulated pores are the main gas transmission channels.\nIn addition, the P-GQDs interlayer spacing obtained from XRD patterns (Fig. R4) and\nHRTEM images (Fig. R5) of P-GQDs indicates that the interlayer gaps also may be\nable to transport gases, so the number of layers is likely to affect the membrane\nperformance. Generally, fewer layers of GQDs are conductive to order and compact\nassembly, while more layers of GQDs are inclined to disordered and loose stacking,\nthereby improving gas permeability. Therefore, the number of GQD layers may\ninfluence the gas separation performance of the P-GQD membrane by affecting the\nratio of the two gas transport channels (stacked pores and interlayer gaps). Finally, this\nwork mainly focuses on developing the proposed strategy to assemble GQDs as\nbuilding blocks into membranes for the first time. Further related work needs to be\nstudied and improved. Therefore, how the number of GQD layers affects the membrane\nstructure requires further detailed characterization.\n4. The alumina substrate was heated to 180 °C. How did the authors confirm that this\ntemperature does not adversely affect the structure of the GQDs?\nResponse: Thank you for this comment. Firstly, GQDs used in this work were\nprepared by thermal decomposition of citric acid at 210 °C, and therefore did not\ndecompose further at 180°C. Furthermore, the thermogravimetric curve of GQDs (Fig.\n2b) shows that only partial water loss occurs at 180 ℃, with no significant weight loss\nor decomposition. Therefore, the spraying temperature of 180 °C would not destroy the\nstructure of GQDs. To further confirm this, we compared the FTIR spectra of the\nsynthesized GQD powders and the GQD membrane sprayed at 180 ℃. As shown in\nFig. R2, the sprayed GQD membrane retained the same active groups of GQDs, such\nas carboxyl and hydroxyl groups.\nFig. R2 FTIR spectra of GQD powders and GQD membrane sprayed at 180 ℃.\nFollowing the reviewer’s comment, the following sentences were added on Page 7 of\nthe revised MS to support the revision of the relevant issue.\n“At this optimized support temperature (180 °C, see Supplementary Fig. 4), GQDs\nremain on the surface without denaturation, maintaining similar chemical groups and\nstructures.”\n5. The authors mention the regulation of angstrom-scale pores, but direct\ncharacterization and explanation of this critical feature are lacking in the article.\nResponse: We sincerely thank the reviewer for the valuable comment. In this study,\nvarying the calcination temperature modifies the degree of crosslinking and\nvolatilization of some light elements, thereby altering the pore size of the P-GQD\nmembrane and resulting in different gas transport behaviors. The pore size distribution\nof P-GQDs prepared at 350 °C was shown in the initial manuscript (Fig. 4e). In addition,\nthe pore size distributions of P-GQDs calcined at 320 °C and 380 °C were further\ncharacterized. CO adsorption isotherms at 273 K were collected and the pore size\n2\ndistributions were determined using the non-local density functional theory (NLDFT)\nmodel. As presented in Fig. R3, P-GQDs exhibit a bimodal ultra-microporous structure\nwith sizes of 0.33-0.40 nm and 0.43-0.70 nm, respectively. The total micropore volume\nincreases with calcination temperature due to enhanced release of volatile species. In\naddition, a fundamental trade-off exists: pores > 0.4 nm become more, whereas the\npores < 0.4 nm required for size-sieving of CO are maximized at intermediate\n2\ntemperature (for instance 350 °C) and diminishes at 380 °C due to pore widening. This\nmechanism demonstrates that strategic tailoring of calcination temperature allows for\nthe manipulation of pore size distribution, thus providing a convenient means to\noptimize the separation of CO and larger molecules.\n2\nadsorption at 273 K.\nFollowing the reviewer’s comment, the above Fig. R3 was added as Supplementary Fig.\n28 in the revised SI, a few sentences were added on Page 17 of the revised MS to\nsupport the revision of the relevant issue.\n“…P-GQDs exhibit a bimodal ultra-microporous structure with sizes of 0.33-0.40 nm\nand 0.43-0.70 nm (Supplementary Fig. 28). The total micropore volume increases with\ncalcination temperature due to enhanced release of volatile species. A fundamental\ntrade-off exists: >0.4 nm pores become more, whereas <0.4 nm pores required for size-\nsieving of CO is maximized at intermediate temperature (for instance 350 °C) and\n2\ndiminishes at 380 °C due to pore widening.”\n6. The cross-linking reaction between pyrolyzed PEI products and GQDs is a crucial\nstep in modulating the membrane structure, yet the description of the reaction\nmechanism remains somewhat vague.\nResponse: We are very grateful to the reviewer for the helpful comment. We\ninvestigated the cross-linking mechanism of PEI pyrolysis products and GQDs by\nprobing the evolution of P-GQD membrane surface groups. In Supplementary Fig. 6,\nwhen temperature is below 230 ℃, neither GQD nor PEI begins to pyrolyze, so the\nfunctional groups hardly change. In the period, only the carboxyl groups of GQDs\ndehydrate to generate anhydride, corresponding to the peak at 1790 cm-1. Until heated\nto 280 ℃, the carboxyl and hydroxyl group of GQDs react with each other to form\nesters, which is shown in the infrared spectrum. The peaks at 1771 cm-1 and 1736 cm-1\nare attributed to the C=O stretching vibration, and the peak at 1185 cm-1 belongs to the\n to the\nC-O stretching vibration of the esters. After PEI starts to pyrolyze, the amino groups of\nsmall molecule amines react with the carboxyl groups of GQDs to form amide, which\nappears in the infrared spectrum at 320 ℃, where 1640 cm-1 represents the C=O of\namide and 1261 cm-1 represents the C-N of amide. In addition, it is clearly seen that the\nC≡N group at 2228 cm-1 appears at 320 ℃, further indicating that small amines from\nPEI have chemically cross-linked onto the GQDs. Simultaneously, the peaks of 2360\ncm-1 and 2340 cm-1 are assigned to CO , attributing to the adsorption of CO by the\n2 2\nnewly introduced N-containing groups. As the heat treatment temperature further\nincreases, the peak intensity of these surface groups gradually decreases, indicating that\nthe content of O, N-containing functional groups has decreased, which is due to the\ncontinuous thermal decomposition of the cross-linked P-GQD membrane, causing the\nloss of partial elements.\nFollowing the reviewer’s comment, we added a few sentences on Page 8 of the revised\nMS and Page 6 of the revised SI to support the revision of the relevant issue.\n“Below 230 °C, neither GQDs nor PEI pyrolyze, and thus there is no significant change\nin functional groups. At 280 °C, the carboxyl and hydroxyl group of GQDs react with\neach other to form esters. After PEI starts to pyrolyze, the amino groups of small\nmolecule amines react with the carboxyl groups of GQDs to form amide.\nSimultaneously, generation of -CN and enhanced adsorption of CO further indicate\n2\nthat small amines from PEI have chemically cross-linked onto the GQDs. As\ncalcination temperature further increases, the content of O, N-containing functional\ngroups has decreased, which is due to the continuous thermal decomposition of the\ncross-linked P-GQD membrane, causing the loss of partial elements.”\n“Below 230 °C, only the carboxyl groups of GQDs dehydrate to generate anhydride,\ncorresponding to the peak at 1790 cm-1. At 280 °C, the peaks at 1771 cm-1 and 1736\ncm-1 correspond to C=O stretching vibration, and the peak at 1185 cm-1 belongs to the\nC-O stretching vibration of the esters. After PEI starts to pyrolyze, the peaks at 1640\ncm-1 and 1261 cm-1 represent the C=O and C-N bonds of amide, respectively.\nSimultaneously, a -CN stretching peak appears at 2228 cm-1, and clear CO asymmetric\n2\nstretching peaks is observed at 2340 cm-1 and 2360 cm-1.”\n7. High-temperature calcination may alter the stacking mode, interlayer spacing, or\ndefect states of the GQDs—physical structural aspects that were not analyzed (e.g., via\nXRD, BET, or HRTEM). It is recommended to supplement the physical structural\ncharacterization of the membranes after calcination to establish a more comprehensive\nstructure–performance relationship.\nResponse: Thank you for your comment and it has been well taken. The structural\ncharacteristics of P-GQDs were examined using XRD and HRTEM. XRD patterns of\nGQDs and P-GQDs calcined at 350 °C display a broad peak at around 20o,\ncorresponding to (002) plane of graphite (Fig. R4). The average d-spacing calculated\nfrom the Bragg’s law is 0.44 nm. HRTEM images of P-GQDs calcined at 350 °C are\nshown in Fig. R5. Clear lattice fringes are observed, with measured lattice spacing of\n0.44 nm and 0.24 nm. These values are assigned to the (002) and (100) plane of graphite,\nrespectively, which are consistent with the XRD results. After calcination, the retention\nof lattice fringes confirms the presence of highly graphited GQDs. Furthermore, the P-\nGQD membrane structure are further visualized, showing that GQD nanoparticles stack\nand interweave to form a three-dimensional network structure, in which the stacked\npores and interlayer voids together contribute to gas transport channels. To assess\nporosity, N adsorption isotherm was measured at 77 K (Fig. R6). However, they fail to\n2\naccess ultramicropores due to size limitations, so CO adsorption at 273 K was used to\n2\nanalyze the micropore distribution in the main text.\nFig. R4 XRD patterns of GQDs and P-GQDs calcined at 350 °C.\nFig. R5 (a, b) HRTEM images of P-GQDs calcined at 350 °C.\nFig. R6 Adsorption isotherm of N at 77 K on P-GQDs.\nSupplementary Fig. 12 and Fig. 13 in the revised SI, and a few sentences were added\non Page 9 of the revised MS and Page 10 of the revised SI to show the structure of the\nas-prepared membranes.\n“XRD and HRTEM were also used to characterize the structural features of the P-GQD\nmembrane, showing that the graphitic structure of GQD particles is preserved within\nthe P-GQD membrane, and that interparticle stacking builds transport pathways\n(Supplementary Fig. 12 and 13).”\n“XRD patterns of GQDs and P-GQDs display a broad peak at around 20°,\ncorresponding to (002) plane of graphite. The average d-spacing calculated from the\nBragg’s law is 0.44 nm.”\n“From HRTEM images of P-GQDs, clear lattice fringes are observed, with measured\nlattice spacing of 0.44 nm and 0.24 nm. These values are assigned to the (002) and (100)\nplane of graphite, respectively.”\n8. The pore size shown in Fig. 4e is presented as a single value, but given the inherent\nsize distribution of the GQDs, the resulting pores would also be expected to exhibit a\ndistribution. Could the authors clarify this?\nResponse: We sincerely thank you for this comment. In this work, CO was used as\n2\na probe molecule to analyze micropore size distribution of P-GQDs. As shown in Fig.\nR7, the pore size is distributed in different ranges. The dense stacking of GQDs and the\ncross-linking with small amine molecules from PEI pyrolysis, as well as the\nvolatilization of some light elements caused by calcination, result in a series of pore\nsize. In the initial manuscript, the pore size analysis method employed only obtained\npores with diameter up to 0.45 nm. The methodology was subsequently refined,\nrevealing the full microporous distribution.\nFig. R7 Pore size distributions of P-GQDs derived from NLDFT analysis of CO\n2\nadsorption at 273 K.\nFollowing the reviewer’s comment, the above Fig. R7 has been utilized as the updated\nFig. 4e.\n9. While using mixed amine compounds from PEI pyrolysis for cross-linking enables\npore size regulation, the precise ratios and reactivity of these components are\nchallenging to control. Could the authors elaborate on the rationale for choosing PEI\npyrolysis over cross-linking with a single small molecule amine?\nResponse: We appreciate this comment from the reviewer. Indeed, it is challenging\nto precisely quantify the amount of small molecule amines that ultimately cross-link\nwith the GQD surface during PEI pyrolysis. The key requirement is to ensure that\nduring the pyrolysis of GQDs to generate pores, these amines are available to react with\nthe active sites on GQD membrane surface to regulate pores. The synergistic weight\nloss, where the release of amines from PEI coincides with the pore generation in GQDs,\nis a fundamental reason for the effectiveness of this cross-linking strategy. However,\nmany single small amines have low boiling points and are prone to volatilize before\nGQDs pyrolysis, thus failing to effectively regulate pores. Therefore, in this work, we\nemployed PEI, which has a higher decomposition temperature, as the small molecule\nsource. In addition, we tried cross-linking the GQD membrane with single amine\nmolecules (e.g., diethylenetriamine). However, the GQD membrane prepared by cross-\nlinking with diethylenetriamine exhibited lower gas permeance and selectivity than the\nPEI-cross-linked membrane (Supplementary Fig. 26). The poor crosslinking effect\nusing single amine molecules may also be related to subtle differences in the porosity\nof GQD stacking, which may result in different crosslinking suitability of single-sized\nmolecules.\nFollowing the reviewer’s comment, we have added a sentence on Page 15 of the revised\nMS to address the poor cross-linking efficiency of single small-molecule amines with\nGQD membranes.\n“…However, their efficacy is somewhat lower than that of PEI-cross-linking due to the\nweight loss mismatch and the inability of a single small molecule to precisely match\nthe subtle variations in the GQDs stacking pores.”\nReviewer #2:\nIn this study, the authors have successfully developed a graphene quantum dot (GQD)\nmembrane with high efficiency for CO separation. By utilizing GQDs as novel\n2\nbuilding blocks, they constructed the fundamental membrane architecture, followed by\nprecise modulation of the pore structure through heat treatment and in situ cross-linking\nwith small-molecule amines. The resulting membrane demonstrates exceptional CO\n2\nseparation performance, which exceeds the industrial separation target. Notably, the\nproposed strategy for pore structure customization not only shows promising potential\nfor C H /C H separation but also provides valuable insights for the rational design of\n3 6 3 8\nmembrane structures tailored to diverse separation systems. Given the significance and\ninnovation of this work, I recommend the manuscript for publication in Nature\nCommunications after addressing the following issues (minor revision):\nResponse: We sincerely appreciate reviewer’s insightful comments on our\nmanuscript. We have carefully revised the manuscript in the light of these comments,\nas detailed below.\n1. A consideration is whether graphene quantum dots (GQDs) have the ability to\npenetrate into the substrate’s internal structure, such as pores or surface defects, and if\nso, how this penetration might alter the substrate’s intrinsic properties.\nResponse: We highly appreciate your helpful comment. Firstly, the surface pore size\nof the Al O substrate used in this study is about 70 nm. Therefore, GQDs smaller than\n2 3\n10 nm inevitably penetrate the substrate. Excessive GQDs infiltration would partially\nblock the substrate’s pores, reducing the substrate’s inherent permeability. Therefore,\nwe used thermal spraying to accelerate solvent evaporation and reduce GQDs\npenetration into the substrate. As can be seen from Supplementary Fig. 4, the thermal\nspraying at 180 °C minimized the penetration of GQDs due to the rapid evaporation the\nsolvent, thereby maintaining the substrate’s original permeability\nFollowing the reviewer’s comment, we have added a few sentences on Page 7 of the\nrevised MS to support the revision of the relevant issue.\n“…the Al O support was heated to 180 °C so that the solvent evaporated rapidly upon\n2 3\ncontact, thereby minimizing the penetration of GQDs and maintaining the substrate’s\noriginal permeability.”\n2. It is strongly recommended that the loading amount of graphene quantum dots onto\nthe support during the spraying process be quantitatively determined. This information\nis essential for establishing a clear correlation between GQD content and membrane\nperformance, as well as for ensuring reproducibility of the fabrication process.\nResponse: We appreciate your valuable comment. During the manual spraying\nprocess, a consistent spray flow rate and distance were maintained to ensure\nreproducible membrane fabrication. Since the spray coverage area exceeded the\nsubstrate dimensions, we calculated the GQDs loading by weighing the substrates\nbefore and after spraying. For different spray volumes, we measured the mass of three\nmembranes in parallel, showing excellent repeatability, as shown in Table R1.\nTable R1. The weight of three parallel membranes for different spray volumes.\nSpray volumes 1 mL 2 mL 3 mL 4 mL\n1.75 mg 2.01 mg 3.54 mg 4.46 mg\nMembrane weight 1.03 mg 2.43 mg 3.60 mg 4.92 mg\n1.26 mg 2.28 mg 3.25 mg 5.08 mg\nAverage weight 1.35 mg 2.24 mg 3.46 mg 4.82 mg\nFollowing the reviewer’s comment, the above Table R1 was added as the updated\nSupplementary Table 5.\n3. Regarding the formation of a continuous GQD membrane on the Al O support,\n2 3\nfurther clarification is needed: does the spraying process alone yield a continuous,\ndefect-free membrane structure, or is subsequent cross-linking with PEI pyrolysis\nproducts necessary to eliminate pinholes, enhance interparticle adhesion, and achieve\nthe required continuity for effective separation?\nResponse: We sincerely thank you for this insightful comment. The GQDs\naggregated into a continuous, dense and gas-impermeable membrane via the thermal\nspraying process, as evidenced by gas permeation tests. The subsequent pyrolysis\nprocess serves dual functions: it generates the necessary pores for gas permeation and\nproduces PEI-derived amine species that cross-link to fine-tune pore size, thereby\nachieving high separation selectivity. We have already mentioned this on Page 7 of the\ninitial manuscript.\n4. Generally, the support and membrane area will affect the separation performance of\nthe membrane. Please provide the specifications of the alumina support used and the\nGQD membrane area tested.\nResponse: Thank you for your comment and the comment is well taken. The\nspecifications of the Al O support are: diameter of 18 mm, thickness of 1 mm, and top\n2 3\nlayer pore size of 70 nm. The diameter of the tested P-GQD membrane is 10 mm.\nFollowing the reviewer’s comment, we added a few sentences on Page 19 and 20 of the\nrevised MS to support the revision of the relevant issue.\n“A porous α-Al O support (top layer pore size of 70 nm, diameter of 18 mm and\n2 3\nthickness of 1 mm) was preheated to 180 °C on a hot plate.”\n“The diameter of the tested P-GQD membrane is 10 mm.”\n5. The observed trend that an increase in feed pressure leads to a decrease in CO\n2\npermeance but an increase in CH permeance, requires a thorough explanation.\n2\npermeance is due to the saturation of CO adsorption caused by high pressure, which is\n2\nattributed to the gas’s Langmuir adsorption behavior, weakening the interaction\nbetween CO and P-GQD membrane. The adsorption of CH is weaker than that of CO .\n2 4 2\nThe increase in feed side pressure increases the CH loading in the membrane, thereby\n4\nenhancing the permeation of CH and hindering the CO diffusion.\n4 2\nFollowing the reviewer’s comment, we added a few sentences on Page 11 of the revised\nMS to explain the pressure-dependent permeance change of CO and CH .\n2 4\n“As shown in Fig. 3a and 3b, both the CO permeance and separation factors of the P-\n4\nincreases. This phenomenon is likely because the elevated pressure increases the CH\n4\nloading in the membrane, thereby promoting CH permeation and hindering CO\n4 2\ndiffusion.”\n6. It is necessary to carefully check the reference format of the manuscript and revise it\nto meet the requirements.\nResponse: Thank you for your suggestion. The reference format of the manuscript\nhas been carefully revised according to the requirements of Nature Communications.\n7. I am wondering if the authors could put a schematic picture in the main text to\nillustrate how to precisely control the pore size of the GQD membranes. This is the\nmost amazing part of this work. Moreover, what is the reproducibility of the membranes?\nResponse: Thank you for your suggestion. A schematic diagram has been\nsupplemented in the revised SI to illustrate the pore size regulation of the P-GQD\nmembranes. The picture is shown below (Fig. R8). In addition, three random\nmembranes tested all showed excellent performance, indicating good reproducibility of\nthe membranes.\nFig. R8 Schematic diagram of the pore size regulation process of P-GQD membranes.\nFollowing the reviewer’s comment and considering the possible pore size regulation\nprocess, we added the above Fig. R8 as Supplementary Fig. 27 in the revised SI. In\naddition, we added one sentence on Page 17 of the revised MS to show the change.\n“Based on the pore formation mechanism and the CO separation results, varying the\n2\ncalcination temperature modifies the degree of crosslinking and volatilization of some\nlight elements, thereby altering the pore size of the P-GQD membrane (Supplementary\nFig. 27).”\nReviewer #3:\nThis work required major revision before it is accepted for publications. Following are\nmy comments:\nResponse: We sincerely appreciate reviewer’s insightful comments on our\nmanuscript and we have carefully revised the manuscript based on these comments.\nComment 1. The introduction is comprehensive and provides a strong basis for using\nGQDs in membrane-based separations. However, it is quite dense with information and\ncould be streamlined. For example, you may consider separating background on\nconventional materials (zeolites, MOFs, etc.) from the motivation for GQDs to improve\nreadability and emphasize the novelty of your approach.\nResponse: Thank you for your suggestion. We have simplified and separated the\nbackground on conventional membranes from the motivation for this study in the\nrevised manuscript to improve readability and clarity. The following shows the\nrevisions:\n“However, the molecular sieving capability of these porous materials, which is primary\nadvantage, is also their fundamental limitation, as the separation efficacy is critically\ndependent on the intrinsic pore size.”\n“Consequently, developing standardized primitive membranes with adjustable pore\nstructures to meet specific permeance and selectivity requirements would greatly\nsimplify membrane manufacturing process and facilitate commercial developments.”\n“Among them, emerging graphene quantum dots (GQDs, size < 10 nm) possess stable\ngraphitized carbon cores and abundant surface functional groups, enabling them to\ntightly self-assemble into three-dimensional networks through cross-linking chemical\nreactions.”\n“Therefore, regulating the crosslinking process is a key process for achieving GQD\nmembranes with tunable pore sizes.”\nComment 2. Mention the concentration of GQDs utilized for gas separation\napplications. In the manuscript, it is mentioned that “a primitive GQD membrane was\nobtained by spraying the GQD sol onto a porous α-Al O support………by spraying a\n2 3\ntotal volume of 2 mL GQD sol, the particles stack into a continuous membrane”. Also,\nin Figure S4 of the SI, SEM images of the surface morphology of the sprayed GQD\nmembrane with different volumes of GQD sol have been presented. Why is 2 mL\nspraying considered as optimum volume for further applications?\nResponse: We appreciate your helpful suggestion. GQDs aqueous sol prepared by\npyrolysis of citric acid was directly used for spraying, and the mass of raw materials\nand the volume of water as solvent were kept the same when preparing different batches\nof GQDs. The concentration of GQDs aqueous sol is approximately 10 mg mL-1. In\naddition, as mentioned in the second question of reviewer #2, since the sprayed\ncoverage area is much larger than the size of the substrate, we calculated the actual\namount of GQDs loaded onto the substrate by weighing the mass of the substrate before\nand after spraying (Table R1).\nThe formation of a continuous and defect-free GQD membrane during the spraying\nprocess is imperative for effective gas separation. Electron microscopy images and\npreliminary performance testing reveal that when the spray volume is insufficient (e.g.,\n1 mL), the GQD membrane is discontinuous and exhibits pinholes, failing to fully cover\nthe substrate’s surface pores. A continuous GQD membrane is achieved after spraying\n2 mL or more. From SEM images of the surface morphology (Supplementary Fig. 5),\nit can be seen that the membrane thickness is directly proportional to the deposited\nvolume. Generally, the membrane thickness affects the diffusion length of gases, so a\nthicker membrane leads to a lower gas permeability. Therefore, a spray volume of 2 mL\nwas chosen as the preliminary optimum to balance membrane integrity and permeance.\nFollowing the reviewer’s comment, we added a few sentences on Page 7 and Page 19\nof the revised MS to support the revision of the relevant issue.\n“The concentration of GQDs aqueous sol is approximately 10 mg mL-1.”\n“After spraying a total volume of 2 mL GQD sol, the particles stack into a thin\ncontinuous membrane, which can more effectively reduce gas transport resistance\ncompared to thicker membranes (Supplementary Fig. 5).”\nComment 3. PEI pyrolysis introduces amino groups for CO affinity and pore regulation;\n2\nthe discussion could benefit from quantitative evidence of interaction. For instance,\nDFT calculations, binding energy analysis would make the mechanistic argument more\nconvincing.\nResponse: We appreciate your insightful comment. The synthesized P-GQD\nmembrane contains complex groups (ester, amide, nitrile), and the uncertainty in their\nspecific contents makes precise DFT calculations particularly difficult. To further\nvalidate the role of nitrogen-containing groups in CO adsorption, the CO adsorption\n2 2\nenthalpy was characterized. In comparison, for S-GQDs calcined at 350 °C alone\nwithout PEI, the zero-coverage adsorption enthalpy of CO was measured to be -25.97\n2\nkJ mol-1, which is lower than that of P-GQDs (-28.14 kJ mol-1) (Fig. R9). The small\ndiscrepancy is likely due to the inherent O-containing groups in S-GQDs, which possess\na moderate binding affinity for CO .\n2\n350 °C. (b) Heat of adsorption (Q ) of CO for S-GQDs calcined at 350 °C.\nst 2\nFollowing the reviewer’s comment, the above Fig. R9 was added as Supplementary Fig.\n22 in the revised SI, and a few sentences were added on Page 14 of the revised MS to\nsupport the revision of the relevant issue.\n“…In comparison, for S-GQDs calcined at 350 °C alone without PEI, the zero-coverage\nadsorption enthalpy of CO was measured to be -25.97 kJ mol-1, which is lower than\n2\nthat of P-GQDs (Supplementary Fig. 22).”\nComment 4. In Figure S1 of the SI, the UV-vis absorption peak of GQDs is not distinct,\nthough it shows a broad band.\nResponse: Thank you for your comment. First, we repeatedly measured the UV-\nvisible absorption spectrum of the GQDs aqueous solution, and the same spectral\nfeature was observed (Fig. R10). Moreover, similar UV-vis absorption spectra have also\nbeen shown in other literature related to carbon dots (Adv. Funct. Mater. 2012, 22, 4732;\nJ. Phys. Chem. C 2016, 120, 1252; J. Am. Chem. Soc. 2006, 128, 7756). The broad\nabsorption peak at 340 nm can be attributed to the abundant functional groups (-COOH,\n-OH, etc.) on the carbon dots surface, which likely form a quasi-continuous energy\ndistribution of surface states. Electronic transitions within this continuum result in the\nobserved featureless, broad adsorption band. The absorption band below 300 nm is\nassigned to π-π* transitions of the conjugated C=C units from the carbon core. The\nabsorption peak broadening can be attributed to the inherent size distribution of carbon\ndots – with different sizes resulting in different energy gaps, as well as the differing\nabsorption characteristics of the crystalline and amorphous phases present in the carbon\ncores.\nFig. R10 UV-visible absorption spectrum of GQDs aqueous sol.\nFollowing the reviewer’s comment, we added a few sentences on Page 5 of the revised\nMS to support the revision of the relevant issue.\n“The UV-vis absorption spectrum (Supplementary Fig. 2) of GQDs displays broad\nbands in the near-UV region, which is the characteristic of surface functional groups\nand carbon core of GQDs.”\nComment 5. The membrane is prepared via thermal spraying + PEI cross-linking. Can\nthis process be scaled beyond small lab samples? Is the method compatible with larger-\narea substrates? A brief comment on scalability and fabrication reproducibility would\nstrengthen the practical relevance.\nResponse: We appreciate your helpful suggestion. In this work, the P-GQD\nmembranes are prepared by thermal spraying, which facilitates industrial scale-up.\nSpray coating technology has already been successfully industrialized in fields such as\nbiomedical coatings and automotive manufacturing, demonstrating its technical\nmaturity and feasibility for large-scale membrane fabrication. Currently, the spray-\ncoating method was employed to prepare large-area membranes, yielding excellent\nseparation performance (J. Membr. Sci. 2021, 638, 119708; J. Membr. Sci. 2023, 671,\n121407; Sep. Purif. Technol. 2025, 378, 134752). To initially demonstrate the scale-up\nprocess, we prepared a large-area GQD membrane with a size of 8×8 cm2 on the Al O\n2 3\nsubstrate by thermal spraying, showing good surface uniformity (Fig. R11). In the\nsubsequent PEI pyrolysis cross-linking process, providing an adequate amount of the\nsmall molecular amines derived from PEI pyrolysis to react with the surface functional\ngroups of GQDs is expected to yield a defect-free membrane. A larger calcination tube\nand a larger membrane testing device are required. In addition, the P-GQD membrane\nis free from the grain boundary problems typical of polycrystalline membranes.\nCombined with the excellent reproducibility of the prepared small-area P-GQD\nmembrane, it further shows that the proposed fabrication method is repeatable.\nFig. R11 Photograph of the large-area GQD flat-sheet membrane.\nFollowing the reviewer’s comment, we added a few sentences on Page 7 of the revised\nMS to support the revision of the relevant issue.\n“…Moreover, the employed spray-coating is a scalable industrial process, which\nsuggests its potential for the deposition of primitive GQD membranes onto large-area\nsubstrates with high reproducibility.”\nComment 6. The XRD patterns of the synthesized materials are not given in the\nmanuscript, which are important for determining the crystallinity of the membranes.\nResponse: We sincerely appreciate your insightful suggestion. XRD patterns of\nGQDs and P-GQDs calcined at 350 °C were conducted in Fig. R4. The XRD patterns\ndisplay a broad peak at around 20o, corresponding to the (002) of graphite, indicating\nan amorphous carbon structure. However, HRTEM images (Fig. R5) of both GQDs and\nP-GQDs reveal well-resolved lattice fringes, indicating the high local crystallinity. The\nbroadening of the XRD peaks possibly arises from their nanometer size, limited number\nof layers, as well as imperfect interlayer stacking. The broad XRD peaks primarily\nreflect microscopic averaging of their limited crystalline domains and possible stacking\ndisorders.\nFig. R4 XRD patterns of GQDs and P-GQDs calcined at 350 °C.\nFollowing the reviewer’s comment, we added a few sentences on Page 9 of the revised\nMS to support the revision of the relevant issue.\n“XRD and HRTEM were also used to characterize the structural features of the P-GQD\nmembrane, showing that the graphitic structure of GQD particles is preserved within\nthe P-GQD membrane, and that interparticle stacking builds transport pathways.”\nComment 7. Provide the Robeson upper bound curve, indicating the synthesized\nmembranes’ performance.\nResponse: Thank you for your comment. Generally, the Robeson upper bound curve\ndescribes the performance limit of polymer gas separation membranes (J. Membr. Sci.\n1991, 62, 165; J. Membr. Sci. 2008, 320, 390). When comparing the synthesized\nmembranes’ performance to the Robeson upper bound, it is necessary to convert the\npermeance (GPU) to permeability (Barrer) based on the membrane thickness. However,\nthe thickness of the P-GQD membrane is difficult to measure accurately. We\nsummarized the performance of some recently published polymer composite\nmembranes to fit the performance range of polymer membranes and found that the\nCO /N and CO /CH performances of the P-GQD membranes exceed the current upper\n2 2 2 4\nlimit (Fig. R12). Therefore, the P-GQD membrane in this study is highly competitive\nin CO separation fields. In the main text, we have evaluated the membranes’\n2\nperformance using industrial separation targets.\nFig. R12 Comparison of P-GQD membrane performance with other polymer\nmembranes.\nComment 8. Stability tests (100 h, pressure cycling) are promising, but many industrial\ngas separation processes involve humidity or mixed contaminants (e.g., SO , H O\n2 2\nvapor). Since GQDs are inherently hydrophilic/hydrophobic depending on cross-\nlinking, more discussion of long-term stability in humid/mixed-gas environments is\nneeded.\nResponse: We appreciate your valuable comment. In response to your suggestion,\nwe further investigated the binary CO /N separation performance of the P-GQD\n2 2\nmembrane under varying test temperatures, humid (3 mol. % steam) and dry conditions.\nDue to the presence of hydrophilic groups in the P-GQD membrane, the introduction\nof water vapor may cause capillary condensation. Therefore, the performance of the P-\nGQD membrane under humid conditions at elevated temperatures has been studied. At\nthe test temperature of 60 ℃, the P-GQD membrane exhibits higher selectivity under\nhumid conditions than under dry conditions (Fig. R13). This enhancement can be\naccounted for by the introduction of water-mediated facilitated transport mechanism\nfor CO . Moreover, the separation performance of the P-GQD membrane can be\n2\nrestored after returning to room temperature and dry gas testing.\nIn addition, the evaluation of P-GQD membrane performance with mixed\ncontaminants, such as SO , is also crucial for practical applications. However, SO is\n2 2\ncorrosive and requires specialized equipment for safe and accurate testing, which is\ncurrently beyond the scope of our laboratory setup. We will investigate the effects of\nSO or other acidic gases in the future work, pending the acquisition of the necessary\n2\napparatus.\nFig. R13 Binary CO /N separation performance of the P-GQD membrane under\n2 2\ndifferent test temperatures and humid environments.\nFollowing the reviewer’s comment, the above Fig. R13 was added as Supplementary\nFig. 19 in the revised SI, and a few sentences were added on Page 12 of the revised MS\nto support the revision of the relevant issue.\n“The performance of the P-GQD membrane was also evaluated under humid conditions.\nWhen exposed to humid feed gas at 60 °C, the P-GQD membrane maintains moderate\npermeance and selectivity, with performance recovery observed after returning to dry\ngas at room temperature (Supplementary Fig. 19).”\nComment 9. The propylene/propane results are impressive, but the mechanistic\nexplanation is underdeveloped. Why exactly does the cut-off emerge at 380 °C? Is it\npurely pore widening, or do changes in surface chemistry (loss of functional groups)\nalso contribute?\nResponse: We sincerely appreciate your insightful comment. To determine whether\nthe membrane surface groups changed, the infrared spectra of P-GQD membranes\ncalcined at temperature up to 380 ℃ were further analyzed. As illustrated in Fig. R14,\nthe initial increase and subsequent decrease in nitrogen-containing groups (e.g., amide,\nnitrile) and ester groups with rising calcination temperature reflects a two-stage\nmechanism: initially, cross-linking between thermally decomposed PEI-derived amines\nand GQDs, followed by volatilization of more light elements. This strategy achieves\nregulation of pore size and adsorption strength by controlling the degree of cross-inking\nand pyrolysis. Pore size distribution analyses of P-GQDs (Fig. R3) reveal that P-GQDs\ncalcined at 380 ℃ exhibit enhanced porosity within the 0.43-0.70 nm range, providing\ndiffusion pathways for propylene molecules. Furthermore, the significant disparity\nbetween the ideal adsorption selectivity (1.98) and the actual separation selectivity (12)\nunderscores a separation mechanism dominated by size-sieving effect in\npropylene/propane separation. Therefore, the effective separation of propylene/propane\ndepends more on the enlargement of pore size.\nFig. R14 (a) FTIR spectra of P-GQD membranes cross-linked at different temperatures.\n(b) The spectra of enlarged E1 area. (c) The spectra of enlarged E2 area.\nFollowing the reviewer’s comment, the above Fig. R14 was added as Supplementary\nFig. 6 in the revised SI, and a sentence on Page 17 of the revised MS to support the\nrevision of the relevant issue.\n“This selectivity is far higher than the IAST-predicted adsorption selectivity (1.98,\nSupplementary Fig. 29), underscoring the dominant role of molecular sieving due to\npore widening.”\nComment 10. You demonstrate strong stability over 100 h, which is impressive.\nHowever, industrial applications often require much longer-term stability (weeks to\nmonths). Even if such tests are not feasible now, discussing potential long-term\nchallenges (e.g., fouling, plasticization, or hydrothermal stability) would make the\nconclusions more compelling for application-focused readers.\nResponse: We are grateful to your valuable comment. In industrial membrane\napplications, challenges such as fouling, plasticization, and hydrothermal aging can\ncritically impact long-term stability. The following discussion is about how the P-GQD\nmembrane in this study copes with these challenges.\nFirstly, the application of membranes faces distinct challenges from specific\ncontaminants: for example, NO and SO in CO /N flue gas separation; and heavy\nx x 2 2\nhydrocarbons as well as H S in CO /CH natural gas purification, respectively. The\n2 2 4\nfabricated P-GQD membrane possess a smooth and hydrophobic surface, which can\nreduce foulants adhesion. However, the partial chemical groups may be susceptible to\ndegradation by acidic gases. Consequently, dependable long-term performance in\nindustrial applications will likely depend in part on robust pretreatment systems for\npollutant removal.\nAdditionally, the P-GQD membrane exhibits a rigid graphitized carbon structure and\nmoderate adsorption of CO , making the membrane less susceptible to plasticization.\npressure testing and showing a slight improvement in separation selectivity during the\nlong-term stability test.\nLastly, the presence of water vapor and high temperature also represent critical\nchallenges in the application of flue gas membranes. When the feed gas containing\nwater vapor is introduced, high permeance of the P-GQD membrane requires a higher\ntemperature to maintain, and the performance stability of the P-GQD membrane at\nelevated temperature remains an area for further improvement.\nIn conclusion, the membrane materials and preparation strategies of the P-GQD\nmembrane proposed in this study are beneficial for industrial scale-up. And the long-\nterm stability under complex industrial working conditions needs to be further\nimproved.\nFollowing the reviewer’s comment, we added a sentence on Page 12 of the revised MS\nto support the revision of the relevant issue.\n“Under actual industrial conditions involving fouling and various gas impurities, the\ndurability of P-GQD membrane needs to be further investigated.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors addressed most of the issues based on my comments, but two points required detailed clarification:\n1. Regarding question 4, I knew that carbon dots did not decompose at 180℃. However, when citric acid was used as the carbon source, the synthesized carbon dots underwent further carbonization over time at high temperatures, forming larger graphitic structures. This is a continuous change process, so changes in the chemical structure also require attention. Nevertheless, FTIR is a qualitative method and cannot provide quantitative data. The authors should indicate the specific content and variations of each element in the XPS, as well as the proportions of specific functional groups after peak fitting. This will help clarify the physicochemical structural evolution of carbon dots during the preparation process and facilitate reader comprehension.\n2. Regarding question 9, Could the reaction mechanism between the pyrolysis products of PEI and the carbon dots be inferred based on the actual pyrolysis products of PEI formed at the experimental temperature? What specific pyrolysis products were generated, and how did they interact with the carbon dots?", "reviewer_3": "I have reviewed the revised manuscript thoroughly. The authors have addressed all the comments raised earlier, and the revisions made are satisfactory. The manuscript is now suitable for publication in its present form."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nWe thank Reviewer #3 for the acceptance of the manuscript. We have carefully\naddressed all points raised by Reviewer #1, and the corresponding revisions have been\nhighlighted in the updated version.\nReviewer #3:\nI have reviewed the revised manuscript thoroughly. The authors have addressed all the\ncomments raised earlier, and the revisions made are satisfactory. The manuscript is\nnow suitable for publication in its present form.\nResponse: We are grateful for your positive evaluation.\nReviewer #1:\nThe authors addressed most of the issues based on my comments, but two points\nrequired detailed clarification:\n1. Regarding question 4, I knew that carbon dots did not decompose at 180 ℃. However,\nwhen citric acid was used as the carbon source, the synthesized carbon dots underwent\nfurther carbonization over time at high temperatures, forming larger graphitic structures.\nThis is a continuous change process, so changes in the chemical structure also require\nattention. Nevertheless, FTIR is a qualitative method and cannot provide quantitative\ndata. The authors should indicate the specific content and variations of each element in\nthe XPS, as well as the proportions of specific functional groups after peak fitting. This\nwill help clarify the physicochemical structural evolution of carbon dots during the\npreparation process and facilitate reader comprehension.\nResponse: Thank you for your suggestion. Indeed, during the direct pyrolysis of\ncitric acid to prepare carbon dots, the sizes of carbon dots increase with increasing\npyrolysis temperature. Therefore, by reasonably controlling the pyrolysis temperature\nand time, we synthesized GQDs with sizes less than 10 nm, which has been confirmed\nby HRTEM (Fig. 1c), AFM (Fig. 1d) and DLS (Supplementary Fig. 1) results.\nIn addition, quantitative data on the chemical structural evolution of carbon dots are\nprovided. We compared the XPS spectra of the synthesized GQD powders and the GQD\nmembrane sprayed at 180 ℃. For GQD powders, the C and O content is 67.69% and\n32.31%, respectively. In the XPS spectrum of GQD membrane sprayed at 180 ℃, due\nto the thin membrane, the signal of the substrate Al O was also detected. The contents\n2 3\nof C, O and Al are 46.51%, 37.46% and 13.16%, respectively, which make it difficult\nto accurately compare the contents of C and O with those of the GQD powders. In\naddition, the proportions of specific functional groups were calculated from the C 1s\nspectrum. As shown in Table R1, the proportion of C=O is almost constant, and the\nproportion of C-O increases, indicating that the carboxyl and hydroxyl groups are\nfurther cross-linked during the 180 ℃ spraying process, resulting in the formation of\nmore esters.\nTable R1. The fitting area ratio of each functional group calculated from C 1s peak\nanalysis of GQD powders and GQD membrane sprayed at 180 ℃.\nC=C C-O C=O\nGQD powders 63.81% 12.57% 23.62%\nSprayed GQD membrane 46.68% 30.17% 23.15%\nFollowing the reviewer’s suggestion, the proportion of each functional group in the\nGQD powders was added to Supplementary Table 1 in the revised SI, and a few\nsentences were added on Page 5 and 7 of the revised MS.\n“The proportions of C=O, C-O and C=C groups are calculated to be 23.62 %,\n12.57 % and 63.81 %, respectively (Supplementary Table 1).”\n“For the as-sprayed primitive GQD membrane, the proportion of C=O is almost\nconstant, and the proportion of C-O increases, indicating that the carboxyl and hydroxyl\ngroups are cross-linked (Supplementary Table 1).”\n2. Regarding question 9, Could the reaction mechanism between the pyrolysis products\nof PEI and the carbon dots be inferred based on the actual pyrolysis products of PEI\nformed at the experimental temperature? What specific pyrolysis products were\ngenerated, and how did they interact with the carbon dots?\nResponse: We thank the reviewer for this comment. The thermal decomposition\nproducts of PEI have been investigated in some literatures (J. Anal. Appl. Pyrolysis\n2023, 169, 105838; J. Appl. Polym. Sci. 2022, 139, e51657). PEI starts to decompose\nin an inert atmosphere at 300 ℃, mainly involving the cleavage of C-N bonds. Below\n400 ℃, the main substances generated include ethylenediamine, diethylenetriamine,\npiperazine, etc. These small molecule amines undergo cross-linking reactions with the\nactive carboxyl and ester groups on the carbon dot surface, thereby regulating the pore\nstructure. The formation of amides was confirmed in the infrared spectrum shown in\nSupplementary Fig. 6, which also demonstrated that the introduction of N-containing\ngroups enhanced CO adsorption. In addition, too low PEI dosage resulted in low\n2\nseparation selectivity of the P-GQD membrane (Supplementary Table 2), which\nindicates that the cross-linking degree of the carbon dot surface is insufficient to form\nsuitable pores. Furthermore, we also employed single small molecule amines such as\ndiethylenetriamine for crosslinking, however, the resulting membrane exhibited\ninferior performance compared to those fabricated using PEI (Supplementary Fig. 26).\nThis observation suggests that molecules with a uniform size may be unable to\nadequately match the differential porosity arising from the stacking of carbon dots.\nIn summary, the reaction mechanism between the pyrolysis products of PEI and carbon\ndots can be inferred: an appropriate amount of small molecule amines crosslinks with\nthe functional groups on the carbon dots to fill the gaps, accompanied by the\nvolatilization of partial light elements, resulting in the pore structure with preferential\nCO permeation.\nrevised SI.\n“It is inferred that multi-sized amine molecules derived from PEI pyrolysis undergo\ncrosslinking reactions with carboxyl and ester groups on CQDs. This allows for\nimproved matching of the differential porosity arising from GQDs stacking, combined\nwith the volatilization of partial elements during calcination, which together regulate\nthe pore structure.”"}, {"round": 3, "reviewer_comments": {"reviewer_1": "This article can be accepted."}, "author_response": ""}]

Decision letter