Synergetic hydrogen-bond network of functionalized graphene and cations for enhanced atmospheric water capture

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The presence of different interfacial components, such as cations or functional groups, shape the physical and chemical properties of the hydrogen bond network. Understanding such interfacial hydrogen-bond networks is essential for a large range of applications and scientific questions. To probe the interfacial hydrogen-bond network, atmospheric water capture is a powerful tool. Here, we experimentally observe that a calcium ion on a calcium-intercalated graphene oxide aerogel (Ca-GOA) surface captures 2.7 times more water molecules than in its freestanding state. From density functional theory (DFT) calculations, we uncover the synergistically enhanced hydrogen-bond network of the calcium ion-epoxide complex due to significantly larger polarizations and hydrogen bond enthalpies. This study reveals valuable insights into the interfacial water hydrogen-bond network on functionalized carbon-cation complexed surfaces and potential pathways for future atmospheric water generation technologies. Physical sciences/Materials science/Nanoscale materials/Graphene Physical sciences/Chemistry/Materials chemistry/Graphene Figures Figure 1 Figure 2 Figure 3 Introduction Under humid or aqueous conditions, interfacial water molecules are ubiquitous in nature and technology. The solid-liquid interface is vital in countless physical, chemical, or biological processes and applications 1 – 4 . One important factor to understand the structural properties of interfacial water is the hydrogen bond network at the interface, which is notoriously difficult to probe 1 , 3 . Recent studies revealed an exciting way to study the interfacial hydrogen network within MOFs 5 , 6 and graphene capillaries 7 via atmospheric water capture (AWC) 8 – 10 . It is now appealing to further broaden the use of this methodology. Firstly, extent AWC beyond the specific case of MOFs or the perfect graphene plane as the solid adsorbent. These materials consider special cases which have few technological and natural analogues. In contrast, functionalised carbon in humid or aqueous environments are ubiquitous and thus relevant in many systems. For example, the hydrophilic groups in lipid bilayers or DNA strands 11 , 12 as well as in hydrophilic polymer membranes 13 , 14 , graphene oxide-based membranes 15 – 19 or single atom catalysts 20 , 21 all obtain carbon-oxygen interfaces with water. All these examples exist in aqueous environments that contain cations. However, there is lack of studies looking at the carbon-oxygen interface with water in the presence of cations despite its enormous fundamental and technological relevance. Hence, conceiving an AWC study that can investigate the interfacial hydrogen network in functionalised carbon under the presence of cations is highly desirable. Here, we solve this issue by utilising functionalised graphene (graphene oxide) as a representative of functionalised carbon. Graphene oxide (GO) can be readily intercalated with cations which allows probing the interfacial hydrogen network of functionalised carbon under the presence of cations via AWC. Graphene oxide is composed of a graphene plane along with various oxygen functionalities 22 – 26 . The presence of functionalised areas gives rise to stable dipole sites and development of a hydrogen bond network with water molecules 15 , 16 . Water transport via GO based materials has been widely studied in experiments and simulations to better understand its surface interaction with water molecules 27 – 34 . Yet, direct experimental evidence is still limited for progressive understanding of the hydrogen-bond network on the GO surface. The most recent water transport study highlighted that intercalated cation on GO surface causes controllable friction with water molecules via hydrogen bondings 16 . The observations suggest that intercalated cations may enrich the hydrogen bond network of water molecules on GO surface. With that, cation intercalated GO offers a unique platform to investigate the hydrogen network of functionalized carbon in a cationic aqueous solution, which is highly relevant in many fields. In this study, we use AWC to reveal that the interaction of cation and oxygen functionalised carbon induces synergistic enhancement of the interfacial hydrogen bond network in aqueous environment. Here, we choose calcium ions due to its outstanding atmospheric water capture ability 35 . This allows us to sensitively detect changes in the hydrogen-bond network in GO due to the presence of the cations. We synthesized calcium-intercalated GO (Ca-GO) in aerogel form and measured the atmospheric water capture capability of the as prepared material. Surprisingly, we experimentally observed that calcium-intercalated GO aerogel (Ca-GOA) presents significantly different water uptake ability than original GO and CaCl 2 . Via further in-depth experimental analysis and computational simulations, this study reveals an enhanced water hydrogen-bond network which is governed by a so far undiscovered synergistic enhancement between oxygen functionalities of GO and hydrated cations. We find that this strong hydrogen-bond network gives rise to new AWC technology using GO-based materials. Results Based on our aim to investigate the functionalised carbon-cation interfacial water hydrogen-bond network, we prepared GO/CaCl 2 aerogel via solution intercalation method 36 . The synthesis procedure is shown in Fig. S1 a and described in detail in the methods section. The as synthesized samples are marked as GOA for graphene oxide aerogel and Ca-GOA for Ca 2+ intercalated GO aerogel respectively. Exemplary images of GOA and Ca-GOA are displayed in Fig. S1 b-d. We characterised the structure and chemical properties of the samples before and after intercalation. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) images are shown in Fig. 1 a-c and Fig. S2 b-c (see supplementary note 2), illustrating the sample morphology. The synthesized Ca-GOA samples display a porous structure similar to the GOA sample with typical wrinkled large surface area. X-ray photoelectron spectrometry (XPS) reveals that the carbon to oxygen ratio (C/O) of GO remains constant at ~ 2 before and after intercalation (Fig. 1 d). C1s XPS spectra curves further suggest that Ca-GOA (Fig. 1 f) have similar carbon-carbon and carbon-oxygen bonds composition compared to GO samples (see Fig.S3a-d in supplementary note 3). The XPS survey and atomic percentage of each element for both GO and Ca-GOA are shown in Fig. 1 f. Around 5.1% of calcium atoms are detected on the Ca-GOA surface, while in the original GO, no calcium was detected. This confirms the successful intercalation of Ca-ions into the Ca-GOA samples. It is noted that a small number of chlorine and sulfur atoms are also detected on Ca-GOA however are not the main concern in this study. To investigate the hydrogen-bond network in the Ca-GOA, we performed AWC measurements as described in the methods section and compared the results to literature values from our own studies and others 35 , 37 . Figure 2 a-c show the recorded and reported adsorption isotherms. The water uptake (m/m 0 ) of Ca-GOA was recorded with varying relative humidity (RH%) at constant room temperature (298K). The relative humidity is then converted into the partial pressure of water vapor to visualise the isotherm. The results show that AWC ability of Ca-GOA samples is significantly improved to up to 2.1 g/g from pure GO samples (GOM) with 0.5 g/g, which indicates the enriched hydrogen network on Ca-GOA surface. It should be noted that the referred GO adsorption curve was measured for GO membranes (GOM) and not in the form of aerogel. However, we ruled out the influence of materials morphology for AWC by showing that GOM and pure GOA have similar AWC ability (supplementary note 3). Further analysis of the isotherms allows us to get a deeper understanding of interfacial water molecules during the AWC. All three different materials, CaCl 2 35 , GOM 37 and our measured data of Ca-GOA, fit well under the BET model 38 , 39 . However, the fitted BET equations are notably different from each (see supplementary note 4 for fitting details). The degree of the polynomial function of the BET equation, represents the number of adsorbed water molecule layers ( n ) on the surface of the material 35 , 39 , 40 . Here, n = 7 for CaCl 2 , n = 4 for GOM and n = 6 for Ca-GOA. Hence, the intercalation of Ca into Ca-GOA results in a stronger hydrogen network on surface compared to pure GO. In other words, the intercalation of cation enhances the interfacial water hydrogen network at the oxidized carbon surface, however the mechanism of such enhancement is still unclear. Based on the BET model fitting analysis, we established that the Ca-intercalation offers strong adsorption sites for water molecules on the Ca-GOA surface. If that is the case, the adsorption isotherm should follow the Freundlich model, as it represents a heterogeneous surface adsorption process 41 . In particular, Freundlich model describes a material with special adsorbing sites which are heterogeneously distributed on the adsorbent surface 42 – 44 . In this case, the Ca atoms are the strong water-attracting adsorbing sites, well-dispersed on the GO plane, allowing coverage of the whole surface with several layers of water molecules 45 . As shown in Fig. 2 d-f, the water adsorption isotherm of Ca-GOA can be fitted in high agreement with the typical Freundlich isotherm model, while being less in agreement with GO and CaCl 2 . The detailed information for Freundlich model fitting is shown in supplementary note 5. The trend of water molecules per oxygen and calcium atom of GOM, GOA, Ca-GOA and CaCl 2 versus the water partial pressure are shown in Fig. 2 g-h. The uptake of water molecules per oxygen atom on Ca-GOA is up to 8 times higher than that of GOM and GOA at all ambient environments. Surprisingly, the water uptake per calcium ion, is up to 2.7 times higher in Ca-GOA compared to CaCl 2 at the highest water partial pressure (Fig. 2 i). Here, we can conclude that the water uptake per calcium and oxygen atom in Ca-GOA is much higher than in the individual materials, CaCl 2 and GOA/GOM. Keeping in mind that the Ca-sites were identified as the main contributor to the water uptake in Ca-GOA, it is striking to see that the water uptake per calcium is 2.7 times higher in the Ca-GOA compared to CaCl 2 . This strongly suggests that this enhancement is linked to an interplay between the functional groups of GO and the intercalated cations to form a synergistically enhanced hydrogen-bond network. To investigate this hypothesis, we further performed density functional theory (DFT) calculations. We examine the hydrogen bond properties between oxygen functionalities and water molecules with and without the existence of calcium ions using DFT calculation. We select a graphene plane with a bare epoxide group as a simplified model of a GO nanoflake surface as shown in Fig. 3 a. Epoxide and hydroxide groups are typical oxygen functionalities on the GO basal plane and have been experimentally confirmed to have a strong hydrogen bonding interaction with water molecules 32 , 46 . The DFT calculations were performed at the PBE0-D3BJ/def2-QZVPP level of theory 47 – 49 . Figure 3 b-c indicates the process of the epoxide hydrogen bonded to one and two water molecules. We obtain hydrogen bond distances and energies typical of moderate hydrogen bonds 50 – 52 . Namely, both water molecules hydrogen bonded to the epoxide group with a distance range of 1.96–1.99 Å and enthalpy of − 14.8 kJ/mol and − 13.4 kJ/mol for the first and second water molecule, respectively. However, when the hydrated calcium cation is bound to the epoxide group, the hydrogen bonding network presents significantly different properties. We note that when the epoxy oxygen is coordinated to the Ca cation, the oxygen is bound to the GO surface via one C–O bond, as illustrated in Fig. 3 d-e. We note that the bonding situation in Fig. 3 d-e also represents the bonding of a hydrated Ca cation to a C–O functional group on the GO surface 53 . Hereinafter, the oxygen connected to the GO surface with a single C–O bond will be referred to as the GO oxygen, rather than the epoxy oxygen. With the presence of calcium cation, our calculations suggest that the GO oxygen is coordinate to the hydrated calcium cation. This dramatically enhances the hydrogen-bond network surrounding GO oxygen. Figure 3 d presents a scenario when one water molecule is hydrogen bonded to both the GO oxygen and the hydrated calcium ion. Compared with the system in Fig. 3 c, this water molecule is now hydrogen bonded via three hydrogen bonds, one with the GO oxygen and two with the hydrating water molecules around the calcium ion. As shown in Fig. 3 d, the hydrogen bond formed with the GO oxygen (1.546 Å) is significantly shorter than the hydrogen bonds formed with the water molecules hydrating the calcium cation (1.699 and 1.725 Å). Furthermore, this hydrogen bond is also significantly shorter than that formed between the water molecule and bare epoxide group presented in Fig. 3 b. Similarly, in Fig. 3 e, we further show the scenario when the GO oxygen is hydrogen bonded to a second water molecule. The calculation shows that the length of the hydrogen bond between the GO oxygen and the second water molecule is also largely shortened, from 1.982 Å (Fig. 3 c) to 1.689 Å (Fig. 3 e). From the hydrogen bond enthalpy point of view, our calculations show that the hydrogen bonding between the GO oxygen and water molecules are highly reinforced by the hydrated calcium ion. In the scenario of the bare epoxide group on the GO surface, the hydrogen bond enthalpy between the oxygen and water molecules is around − 14 kJ/mol, as mentioned above. However, in the presence of the hydrated calcium ion, the hydrogen bond enthalpy between the GO and water molecules in the system shown in Fig. 3 d increases to as much − 66.7 kJ/mol and in Fig. 3 e comes to − 52.9 kJ/mol. Thus, the H-bond enthalpy in the presence of the hydrated calcium ion is about 3–4 times higher than that of a bare system. Both the increased hydrogen bond enthalpy and shorter hydrogen bond lengths are attributed to both the larger hydrogen bond network and stronger hydrogen bonding acceptor strength of the calcium ion decorated GO oxygen. We further investigate the hydrogen bonding acceptor properties of the GO oxygen based on the atomic polar tensor (APT) charges 54 , 55 . The results are shown in Table 1. We obtain the following APT charges of the epoxide oxygen, − 0.40 e , − 0.47 e , and − 0.51 e in the system shown in Fig. 3 a-c, respectively. As expected, the negative charge on the oxygen increases with the number of hydrogen bonds it is involved in. The charge on the hydrogen and oxygen atoms of the water molecules remains relatively constant for systems 3b and 3c. Namely, they range between + 0.23 e and + 0.27 e for the hydrogen and between − 0.45 e and − 0.49 e for the oxygen. Coordination of the hydrated calcium cation to the GO oxygen results in a dramatic change in the oxygen charge. In particular, the coordination of the hydrated calcium cation increases the negative charge on the GO oxygen from − 0.40 e to as much as − 0.86 e . This significant increase in negative charge on the GO oxygen makes it a much stronger hydrogen bond acceptor. Accordingly, the H-bond distance with the water molecule is shortened from 1.965 Å (Fig. 3 b) to 1.546 Å (Fig. 3 d). We note that the later hydrogen bond distance represents an exceptionally short hydrogen bond for a HOH•••O system (i.e., a water molecule coordinated to an oxygen atom) 50 , 51 . It also reveals that coordination of calcium ion to the GO oxygen alters the atomic charges on the hydrogen and oxygen of the hydrogen-bonded water molecules. For example, for the systems depicted in Fig. 3 b and 3 d, the positive charge on the hydrogen increases from + 0.27 e to + 0.37 e and the negative charge on the oxygen increases from − 0.49 e to − 0.63 e (see Table 1). Remarkably, this demonstrates significant medium-range effects of the hydrated calcium cation on the charge of an oxygen centre to which it is bound via a hydrogen bond network (i.e., not covalently bound). The above results indicate that the hydrogen bonding ability of one epoxide group on the GO surface is enhanced by the coordination of the hydrated calcium ion. Such enhancement was shown via the increasing hydrogen bond acceptor strength of GO oxygen and additional hydrogen bonding interactions with the first hydration sphere of calcium cation. It is evident, both theoretically and experimentally, that epoxide groups on a GO surface tend to cluster in islands rather than be uniformly distributed across the surface 26 , 56 . This leads to a natural question, whether this single hydrated calcium cation can interact with more than one epoxide group on GO surface. Table 1 Atomic polar tensor (APT) charge ( q ) in a.u. involved in the hydrogen bonds and on the Ca atom for the systems in Fig. 3 . Model q (O water ) q (H water ) q (O epoxy ) q (Ca) 3a N/A * N/A –0.40 N/A 3b –0.49 + 0.27 –0.47 N/A 3c –0.48, − 0.45 + 0.26, + 0.23 –0.51 N/A 3d –0.63 + 0.37 –0.84 + 1.34 3e –0.61, − 0.60 + 0.40, + 0.35 –0.84 + 1.25 * N/A corresponds to the absence of atomic polar tensor charge. Table 2 Comparison between the hydrogen binding enthalpies at 298 K ( ∆ H 298,bind , in kJ/mol) for the systems in Fig. 3 obtained in the solid state and in bulk aqueous solution. Model ∆H 298, bind (kJ/mol) Solid state Aqueous solution 3a N/A * N/A 3b –14.8 –6.8 3c –13.4 –5.3 3d –66.7 –42.7 3e –52.9 –32.9 * N/A corresponds to the absence of hydrogen binding enthalpy. We further investigate the system with two epoxides on a graphene plane coordinated to a single hydrated calcium ion (see supplementary note 7). Each of the two oxygens on the GO surface is able to hydrogen bond to two water molecules. For the first three water molecules, we obtain binding enthalpies that are similar to those obtained from the functionalized systems in Fig. 3 , namely 70.2, 58.5, and 63.6 kJ/mol, respectively. For the fourth water molecule, we obtain a lower binding enthalpy of 29.9 kJ/mol; this reduction is partly attributed to two (rather than three) hydrogen bonds in which this water molecule is involved in (supplementary note 7). Importantly, all these binding enthalpies are significantly larger than those obtained from the systems with the absence of hydrated calcium cation (Fig. 3 b-c). With the results above, we can now further optimize the hydrogen bond enthalpies from the perspective of the experimental conditions. This is because the calcium intercalated GO surface in the experimental settings is expected to be intermediate between the solid state and a bulk aqueous solution. It is well-established that hydrogen bond strengths are influenced by the effect of the solvent. In particular, the H-bond strength with the GO surface decreases with the polarity of the medium in the order of solid-state > non-polar solvents > polar solvents. Thus, the calculated hydrogen bond enthalpies above, which do not include solvent corrections, are expected to represent the upper limits for the experimental setting. Therefore, it is instructive to calculate the hydrogen bond enthalpies in bulk aqueous solution to obtain lower limits for the hydrogen-bond enthalpies. For this purpose, we use the conductor-like polarizable continuum model (CPCM) 57 , which has been found to provide good performance for aqueous solution 58 , 59 . The solvation corrections reduce the hydrogen bond enthalpies for the unfunctionalized GO models by a factor of \(\sim\) 2.5, whereas they reduce the bond enthalpies for the Ca-functionalized GO models by a factor of \(\sim\) 1.5. As shown in Table 2 , the inclusion of the solvation corrections widens the gap between the hydrogen bond enthalpies of the GO models with and without coordination of the calcium ion. Considering that the GO surface in the experimental settings is expected to be intermediate between the solid state and a bulk aqueous solution, this is strong evidence to explain our experimental observation. Discussion Here, we uncover the synergistic hydrogen-bond network of functionalised carbon in the presence of a hydrated cation. Both, our experimental and computational results show a strong increase in hydrogen bond strength in the system of an epoxy functional group in close range to a hydrated calcium ion on graphene plane. Via AWC experiments we observed that the water uptake per calcium is dramatically increased, by up to a factor of 2.7 times higher in Ca-GOA compared to pure CaCl 2 . Similarly, the water uptake per oxygen of GOA is dramatically increased by a factor of 8 after intercalation of Ca-ions. Via extensive DFT calculations, we uncover that the system of hydrated calcium ion and epoxy functional group on graphene plane enhances the overall binding strength of the hydrogen bond network. Particularly, the calcium ion increases the charge polarisation of the oxygen and hydrogen atoms of the C-O bond and water molecules. This leads to a higher enthalpy and shorter hydrogen bond lengths explaining the experimentally observed enhancement of water uptake per calcium ion. This study holds significance for numerous systems where hydrated ions are in proximity to carbon-based functional groups. As these are omnipresent in nature and technology, our study may help to bring a new perspective to a wide range of natural phenomena and technology applications that involve our described model system. Remarkably, we show that the AWC ability of GO can be enhanced to similar levels as pure CaCl 2 in terms of water uptake per gram. This may open new exciting opportunities to utilise Ca-GOA as a powerful desiccant in atmospheric water generation and energy efficient dehumidification. Methods Materials The GO solution (15 mg/ml) were prepared via Hummer’s method and were supplied by NiSiNa Materials Japan. Calcium chloride (CaCl 2 ) dihydrate powder was purchased from Sigma Aldrich. Synthesis of aerogels The Ca-GO solution was prepared by mixing predetermined volume of CaCl 2 salt solution and GO solution, followed by magnetic stirring for 30 mins at 1000 rpm. The as prepared solution was then freeze dried using a vacuum freeze drier at -60 \(℃\) to synthesize Ca-GOAs. The GOA samples were prepared by freeze drying the GO solution without further modification. All prepared aerogel samples were stored in vacuum condition until experimental measurements. Synthesized samples are shown in supplementary note 1 (Fig. S1 c-d). Characterization of aerogels The X-ray photoelectron spectrometer (XPS, Thermo Scientific, UK ESCALAB250i) with mono-chromatic Al K alpha (energy 1486.68 eV) X-ray source was conducted to investigate the chemical composition of the synthesized aerogels. The C1s peak (284.5 eV) of graphite was used as a reference. The surface morphologies of the aerogels were visualized using a field-emission scanning electron microscope (FEI Nova NanoSEM 230 FE-SEM) with an energy dispersive spectroscopy (EDS) detector (Bruker SDD-EDS) for mapping the qualitative elemental composition. Atmospheric water capture tests The atmospheric water capture (AWC) tests of GOAs and Ca-GOAs were caried out in a custom-designed humidity controlling system (see supplementary note 8). The weight changes of the aerogels were continuously recorded every 10 seconds using a computer-controlled mass balance during the AWC process. The humid environment was maintained stable overnight ( \(\text{R}\text{H}\pm 3\%\) ) before AWC tests. Experiments were repeated more than 2 times to study the standard deviation. Computational details Density functional theory (DFT) calculations were performed to gain further insights into the experimental findings using the Gaussian 16 software 60 . All geometries were fully optimized using the PBE0 exchange-correlation functional 47 in conjunction with def2-SVP basis set 48 (see supplementary note 9). Empirical dispersion corrections 49 are included using the Becke–Johnson potential (denoted by the suffix D3BJ) 61 . Zero-point vibrational energy and enthalpic temperature corrections have been obtained from these calculations. The equilibrium structures were verified to have all real harmonic frequencies, confirming they are local minima on the potential-energy surface. The final electronic energies were refined using the PBE0-D3BJ functional in conjunction with the much larger quadruple-ζ def2-QZVPP basis set 48 . Bulk solvent effects in aqueous solution were also considered using the conductor-like polarizable continuum model (CPCM) continuum solvation model 57 . Declarations Acknowledgment The authors acknowledge the funding for supporting this project from Vesi Water Pty Ltd. X.R. acknowledges the UNSW UIPA Scholarship. The authors acknowledge the staff from Mark Wainwright Analytical Centre at UNSW for technical assistance on sample characterizations using SEM and XPS. V.Q. acknowledges the funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska Curie Grant Agreement No. 101066462. The present work was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. We gratefully acknowledge the system administration support provided by the Faculty of Science, Agriculture, Business and Law at the University of New England to the Linux cluster of the Karton group. D.V.A. and K.S.N. are supported by the Ministry of Education, Singapore, under its Research Centre of Excellence award to the Institute for Functional Intelligent Materials (I-FIM, project no. EDUNC-33-18-279-V12). Xiao Sui conducted this work when she was a research scientist in UNSW. Competing interests X.R., X.S., T.F., D.J., L.O., and R.J. are co-inventors of a PCT application (No. PCT/AU2022/051080). The authors declare no other competing interests. Author contributions R.J. conceived the idea and developed the project with L.O. X.R., and X.S. conducted the experiments. X.R. analysed the data with T.F., R.J., and L.O. A. K. led the computational simulations and contributed to the analysis of results and to develop the mechanism. 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Chemical Engineering Journal 156 , 2–10 (2010). Swenson, H. & Stadie, N. P. Langmuir’s Theory of Adsorption: A Centennial Review. Langmuir 35 , 5409–5426 (2019). Freundlich, H. Über die Adsorption in Lösungen. Zeitschrift für Physikalische Chemie 57U , 385–470 (1907). Kalam, S., Abu-Khamsin, S. A., Kamal, M. S. & Patil, S. Surfactant Adsorption Isotherms: A Review. ACS Omega 6 , 32342–32348 (2021). Buchsteiner, A., Lerf, A. & Pieper, J. Water Dynamics in Graphite Oxide Investigated with Neutron Scattering. J Phys Chem B 110 , 22328–22338 (2006). Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys Rev Lett 77 , 3865–3868 (1996). Weigend, F. & Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Physical Chemistry Chemical Physics 7 , 3297 (2005). Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 132 , (2010). Steiner, T. The Hydrogen Bond in the Solid State. Angewandte Chemie International Edition 41 , 48–76 (2002). Sigala, P. A. et al. Determination of Hydrogen Bond Structure in Water versus Aprotic Environments To Test the Relationship Between Length and Stability. J Am Chem Soc 137 , 5730–5740 (2015). Weinhold, F. & Klein, R. A. What is a hydrogen bond? Mutually consistent theoretical and experimental criteria for characterizing H-bonding interactions. Mol Phys 110 , 565–579 (2012). Saito, A., Obata, S. & Nishina, Y. Uniform coating of magnesium oxide crystal with reduced graphene oxide achieves moisture barrier performance. Appl Surf Sci 573 , 151483 (2022). Cioslowski, J. A new population analysis based on atomic polar tensors. J Am Chem Soc 111 , 8333–8336 (1989). De Proft, F., Martin, J. M. L. & Geerlings, P. On the performance of density functional methods for describing atomic populations, dipole moments and infrared intensities. Chem Phys Lett 250 , 393–401 (1996). Shin, D. S. et al. Distribution of oxygen functional groups of graphene oxide obtained from low-temperature atomic layer deposition of titanium oxide. RSC Adv 7 , 13979–13984 (2017). Cossi, M., Rega, N., Scalmani, G. & Barone, V. Energies, structures, and electronic properties of molecules in solution with the C‐PCM solvation model. J Comput Chem 24 , 669–681 (2003). Takano, Y. & Houk, K. N. Benchmarking the Conductor-like Polarizable Continuum Model (CPCM) for Aqueous Solvation Free Energies of Neutral and Ionic Organic Molecules. J Chem Theory Comput 1 , 70–77 (2005). Skyner, R. E., McDonagh, J. L., Groom, C. R., van Mourik, T. & Mitchell, J. B. O. A review of methods for the calculation of solution free energies and the modelling of systems in solution. Physical Chemistry Chemical Physics 17 , 6174–6191 (2015). Frisch, M. J. et al. Gaussian 16, Revision C.01. Preprint at (2016). Grimme, S., Ehrlich, S. & Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J Comput Chem 32 , 1456–1465 (2011). Additional Declarations Yes there is potential Competing Interest. X.R., X.S., T.F., D.J., L.O., and R.J. are co-inventors of a PCT application (No. PCT/AU2022/051080). Authors declare no other competing interests. Supplementary Files SIRenetal.docx Supporting Information file SItableRenetal.pdf DFT Data Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4010517","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":281861728,"identity":"4ca9c031-7318-4e89-80e5-2f83b29f394e","order_by":0,"name":"Rakesh Joshi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2ElEQVRIiWNgGAWjYHCCBAbGBgk5KIeZeC3GJGlhAGphSGwgWot5A8PDhz93WKRvuN3+TIKhwjqxgf2MAV4tMgcYko15z0jkbrhzxkyC4Ux6YgNPDn4tEgwMadKMbUAtN3LYbjC2HQa6kAgtkj/bJNINbqQ/u8H4D6iF/w1hLRK8bRIJBjcSzG4wNgC1SBCyhRnklzYJw5k3csx/JBxLN26TeFaAXwt7T+LDn2118nw30h8bfKixlu3nT96AVwsDM08CggNisuFXDwLsBwirGQWjYBSMgpENAOifQdZA3ijAAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-7497-9499","institution":"University of New South Wales","correspondingAuthor":true,"prefix":"","firstName":"Rakesh","middleName":"","lastName":"Joshi","suffix":""},{"id":281861729,"identity":"689b3956-0d76-42cf-babc-5e5dfc63f3a8","order_by":1,"name":"Xiaojun Ren","email":"","orcid":"","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Xiaojun","middleName":"","lastName":"Ren","suffix":""},{"id":281861730,"identity":"71f05232-3c99-4787-831e-b740a7aa1c45","order_by":2,"name":"Xiao Sui","email":"","orcid":"","institution":"Harbin Institute of Technology (Weihai)","correspondingAuthor":false,"prefix":"","firstName":"Xiao","middleName":"","lastName":"Sui","suffix":""},{"id":281861731,"identity":"bd2ee5cd-44a0-4fba-a696-c0531e7466ae","order_by":3,"name":"Llewellyn Owens","email":"","orcid":"","institution":"Vesi water Pty Ltd","correspondingAuthor":false,"prefix":"","firstName":"Llewellyn","middleName":"","lastName":"Owens","suffix":""},{"id":281861732,"identity":"03910a1d-c981-40b1-8944-ca0dbb6c1087","order_by":4,"name":"Dali Ji","email":"","orcid":"","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Dali","middleName":"","lastName":"Ji","suffix":""},{"id":281861733,"identity":"05bc9ef5-0cce-4413-a3bf-dafe21722338","order_by":5,"name":"Xinyue Wen","email":"","orcid":"https://orcid.org/0000-0003-3785-2218","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Xinyue","middleName":"","lastName":"Wen","suffix":""},{"id":281861734,"identity":"3136443b-1e28-48d0-8156-68e16c2fbd47","order_by":6,"name":"Yuta Nishina","email":"","orcid":"https://orcid.org/0000-0002-4958-1753","institution":"Okayama University","correspondingAuthor":false,"prefix":"","firstName":"Yuta","middleName":"","lastName":"Nishina","suffix":""},{"id":281861735,"identity":"3bd61ce8-1b41-42c7-89e3-43c5844eaf1c","order_by":7,"name":"Kamal Pant","email":"","orcid":"","institution":"I.I.T Roorkee","correspondingAuthor":false,"prefix":"","firstName":"Kamal","middleName":"","lastName":"Pant","suffix":""},{"id":281861736,"identity":"4221be55-8e5f-413b-970d-8d720d8a1823","order_by":8,"name":"Vanesa Quintano","email":"","orcid":"https://orcid.org/0000-0001-9109-9832","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Vanesa","middleName":"","lastName":"Quintano","suffix":""},{"id":281861737,"identity":"2dc6ce4c-d867-44dd-9168-213cef1a99c2","order_by":9,"name":"Daria Andreeva","email":"","orcid":"https://orcid.org/0000-0003-0273-2064","institution":"National University of Singapore","correspondingAuthor":false,"prefix":"","firstName":"Daria","middleName":"","lastName":"Andreeva","suffix":""},{"id":281861738,"identity":"8a62cb2a-25af-4e6a-bada-fbae2b72034d","order_by":10,"name":"Konstantin Novoselov","email":"","orcid":"https://orcid.org/0000-0003-4972-5371","institution":"National University of Singapore","correspondingAuthor":false,"prefix":"","firstName":"Konstantin","middleName":"","lastName":"Novoselov","suffix":""},{"id":281861739,"identity":"93263f41-5c37-49ae-92f3-54f72b2d1990","order_by":11,"name":"Amir Karton","email":"","orcid":"","institution":"University of New England","correspondingAuthor":false,"prefix":"","firstName":"Amir","middleName":"","lastName":"Karton","suffix":""},{"id":281861740,"identity":"2dbfee2a-6a79-45c3-84a5-eb973ac8dfa2","order_by":12,"name":"Tobias Foller","email":"","orcid":"","institution":"University of New South Wales","correspondingAuthor":false,"prefix":"","firstName":"Tobias","middleName":"","lastName":"Foller","suffix":""}],"badges":[],"createdAt":"2024-03-04 04:40:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4010517/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4010517/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":53181209,"identity":"f7ffcdf1-f3ee-4eeb-9ee5-21ff17e02cbe","added_by":"auto","created_at":"2024-03-21 15:38:50","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":876104,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCharacterization of Ca-GOA. a \u003c/strong\u003eScanning electron microscopy (SEM) image of the overall structure of Ca-GOA. \u003cstrong\u003eb \u003c/strong\u003eSEM image of Ca-GOA showing typical wrinkles. \u003cstrong\u003ec \u003c/strong\u003eSEM image of Ca-GOA with energy dispersive spectroscopy (EDS) showing elemental images of carbon, oxygen, and calcium. \u003cstrong\u003ed\u003c/strong\u003e X-ray photoelectron spectrometry (XPS) survey of GO and Ca-GOA showing C1s, O1s and Ca2p3 peak with carbon/oxygen (C/O) ratio. \u003cstrong\u003ee \u003c/strong\u003eXPS spectra (black curve) showing C1s peak of Ca-GOA sample with peak fitting of C=C/C-C bond at ~284.5 eV, C-O bond (red curve) at ~286 eV and C=O bond (purple curve) at ~288 eV. \u003cstrong\u003ef \u003c/strong\u003eAtomic percentage of\u003cstrong\u003e \u003c/strong\u003eelemental composition including calcium, oxygen and carbon of GO and Ca-GOA samples.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/14194db8f96c06bfa4ee2fb2.png"},{"id":53181208,"identity":"d6d1df67-96a8-4fa1-bab8-d28b24832a70","added_by":"auto","created_at":"2024-03-21 15:38:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":194344,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAdsorption isotherm of CaCl\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e, GOM and Ca-GOA.\u003c/strong\u003e \u003cstrong\u003ea-c \u003c/strong\u003eWater uptake of CaCl\u003csub\u003e2 \u003c/sub\u003e(\u003cstrong\u003ea\u003c/strong\u003e), GOM (\u003cstrong\u003eb\u003c/strong\u003e) and Ca-GOA (\u003cstrong\u003ec\u003c/strong\u003e) under different ambient water partial pressure (kPa). The error bar indicates the standard deviation.\u003cstrong\u003e \u003c/strong\u003eBlack dashed lines illustrate the fractal BET fitting of CaCl\u003csub\u003e2\u003c/sub\u003e, GOM, and Ca-GOA. \u003cstrong\u003ed-f\u003c/strong\u003e Freundlich linear fitting (black dashed line) of CaCl\u003csub\u003e2 \u003c/sub\u003e(\u003cstrong\u003ed\u003c/strong\u003e), GOM (\u003cstrong\u003ee\u003c/strong\u003e) and Ca-GOA (\u003cstrong\u003ef\u003c/strong\u003e). The inset diagram in (\u003cstrong\u003ef\u003c/strong\u003e)\u003cstrong\u003e \u003c/strong\u003eindicates the heterogeneous adsorption behaviours of Freundlich model. \u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2 \u003c/em\u003e\u003c/sup\u003erepresents the coefficient of determination of the fitting. \u003cstrong\u003eg-h\u003c/strong\u003e Comparison of estimated water molecules per oxygen (\u003cstrong\u003eg\u003c/strong\u003e) and calcium atom (\u003cstrong\u003eh\u003c/strong\u003e) between GOM, GOA\u003csub\u003e \u003c/sub\u003eand Ca-GOA under different ambient water partial pressure (kPa). \u003cstrong\u003ei\u003c/strong\u003e Estimation of the highest number of water molecules per oxygen/calcium atom in CaCl\u003csub\u003e2\u003c/sub\u003e, Ca-GOA, GOA and GOM based on the experimental observation of water uptake.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/8e6fa0bb865cfc60e51bebda.png"},{"id":53181211,"identity":"61840f43-9179-4cdf-af76-600662142840","added_by":"auto","created_at":"2024-03-21 15:38:50","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":560969,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGraphene plane-based models for DFT calculations.\u003c/strong\u003e \u003cstrong\u003ea \u003c/strong\u003eSingle epoxide group. \u003cstrong\u003eb-c \u003c/strong\u003eEpoxide group with 1 (\u003cstrong\u003eb\u003c/strong\u003e) or 2 (\u003cstrong\u003ec\u003c/strong\u003e) water molecules. \u003cstrong\u003ed-e\u003c/strong\u003e GO oxygen coordinated to hydrated calcium ion with 1 (\u003cstrong\u003ed\u003c/strong\u003e) or 2 (\u003cstrong\u003ee\u003c/strong\u003e) water molecules. Δ\u003cem\u003eH\u003c/em\u003e\u003csub\u003e298, bind\u003c/sub\u003e represents hydrogen bond enthalpies. Dashed lines indicate hydrogen bond lengths (selected H-bond distances given in Å). \u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/d8cba72312eda691072eae85.png"},{"id":55923094,"identity":"cbdcd5e8-3868-4f74-b43b-4f6f18344097","added_by":"auto","created_at":"2024-05-06 10:44:40","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2094788,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/f619c11a-b582-4b68-9b3b-f2305f24171d.pdf"},{"id":53181210,"identity":"eee543d5-a6e4-43ea-a8af-5a4f003648a4","added_by":"auto","created_at":"2024-03-21 15:38:50","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2647958,"visible":true,"origin":"","legend":"\u003cp\u003eSupporting Information file\u003c/p\u003e","description":"","filename":"SIRenetal.docx","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/c45d51bfe59d99c649f5862c.docx"},{"id":53181212,"identity":"617f1299-d51c-4371-9fe7-e4e5579305d5","added_by":"auto","created_at":"2024-03-21 15:38:51","extension":"pdf","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":221770,"visible":true,"origin":"","legend":"\u003cp\u003eDFT Data\u003c/p\u003e","description":"","filename":"SItableRenetal.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4010517/v1/a58c935cdb74702916d30bb2.pdf"}],"financialInterests":"\u003cb\u003eYes\u003c/b\u003e there is potential Competing Interest.\nX.R., X.S., T.F., D.J., L.O., and R.J. are co-inventors of a PCT application (No. PCT/AU2022/051080). Authors declare no other competing interests.","formattedTitle":"Synergetic hydrogen-bond network of functionalized graphene and cations for enhanced atmospheric water capture","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUnder humid or aqueous conditions, interfacial water molecules are ubiquitous in nature and technology. The solid-liquid interface is vital in countless physical, chemical, or biological processes and applications\u003csup\u003e\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. One important factor to understand the structural properties of interfacial water is the hydrogen bond network at the interface, which is notoriously difficult to probe\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Recent studies revealed an exciting way to study the interfacial hydrogen network within MOFs\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e and graphene capillaries\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e via atmospheric water capture (AWC)\u003csup\u003e\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. It is now appealing to further broaden the use of this methodology. Firstly, extent AWC beyond the specific case of MOFs or the perfect graphene plane as the solid adsorbent. These materials consider special cases which have few technological and natural analogues. In contrast, functionalised carbon in humid or aqueous environments are ubiquitous and thus relevant in many systems. For example, the hydrophilic groups in lipid bilayers or DNA strands\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e as well as in hydrophilic polymer membranes\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, graphene oxide-based membranes\u003csup\u003e\u003cspan additionalcitationids=\"CR16 CR17 CR18\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e or single atom catalysts\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e all obtain carbon-oxygen interfaces with water.\u003c/p\u003e \u003cp\u003eAll these examples exist in aqueous environments that contain cations. However, there is lack of studies looking at the carbon-oxygen interface with water in the presence of cations despite its enormous fundamental and technological relevance. Hence, conceiving an AWC study that can investigate the interfacial hydrogen network in functionalised carbon under the presence of cations is highly desirable. Here, we solve this issue by utilising functionalised graphene (graphene oxide) as a representative of functionalised carbon. Graphene oxide (GO) can be readily intercalated with cations which allows probing the interfacial hydrogen network of functionalised carbon under the presence of cations via AWC.\u003c/p\u003e \u003cp\u003eGraphene oxide is composed of a graphene plane along with various oxygen functionalities\u003csup\u003e\u003cspan additionalcitationids=\"CR23 CR24 CR25\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The presence of functionalised areas gives rise to stable dipole sites and development of a hydrogen bond network with water molecules\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Water transport via GO based materials has been widely studied in experiments and simulations to better understand its surface interaction with water molecules\u003csup\u003e\u003cspan additionalcitationids=\"CR28 CR29 CR30 CR31 CR32 CR33\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Yet, direct experimental evidence is still limited for progressive understanding of the hydrogen-bond network on the GO surface. The most recent water transport study highlighted that intercalated cation on GO surface causes controllable friction with water molecules via hydrogen bondings\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The observations suggest that intercalated cations may enrich the hydrogen bond network of water molecules on GO surface. With that, cation intercalated GO offers a unique platform to investigate the hydrogen network of functionalized carbon in a cationic aqueous solution, which is highly relevant in many fields.\u003c/p\u003e \u003cp\u003eIn this study, we use AWC to reveal that the interaction of cation and oxygen functionalised carbon induces synergistic enhancement of the interfacial hydrogen bond network in aqueous environment. Here, we choose calcium ions due to its outstanding atmospheric water capture ability\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. This allows us to sensitively detect changes in the hydrogen-bond network in GO due to the presence of the cations. We synthesized calcium-intercalated GO (Ca-GO) in aerogel form and measured the atmospheric water capture capability of the as prepared material. Surprisingly, we experimentally observed that calcium-intercalated GO aerogel (Ca-GOA) presents significantly different water uptake ability than original GO and CaCl\u003csub\u003e2\u003c/sub\u003e. Via further in-depth experimental analysis and computational simulations, this study reveals an enhanced water hydrogen-bond network which is governed by a so far undiscovered synergistic enhancement between oxygen functionalities of GO and hydrated cations. We find that this strong hydrogen-bond network gives rise to new AWC technology using GO-based materials.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eBased on our aim to investigate the functionalised carbon-cation interfacial water hydrogen-bond network, we prepared GO/CaCl\u003csub\u003e2\u003c/sub\u003e aerogel via solution intercalation method\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. The synthesis procedure is shown in Fig.\u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003ea and described in detail in the \u003cspan class=\"InternalRef\"\u003emethods\u003c/span\u003e section. The as synthesized samples are marked as GOA for graphene oxide aerogel and Ca-GOA for Ca\u003csup\u003e2+\u003c/sup\u003e intercalated GO aerogel respectively. Exemplary images of GOA and Ca-GOA are displayed in Fig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003eb-d.\u003c/p\u003e\n\u003cp\u003eWe characterised the structure and chemical properties of the samples before and after intercalation. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) images are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea-c and Fig. \u003cspan class=\"InternalRef\"\u003eS2\u003c/span\u003eb-c (see supplementary note 2), illustrating the sample morphology. The synthesized Ca-GOA samples display a porous structure similar to the GOA sample with typical wrinkled large surface area. X-ray photoelectron spectrometry (XPS) reveals that the carbon to oxygen ratio (C/O) of GO remains constant at ~\u0026thinsp;2 before and after intercalation (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed). C1s XPS spectra curves further suggest that Ca-GOA (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ef) have similar carbon-carbon and carbon-oxygen bonds composition compared to GO samples (see Fig.S3a-d in supplementary note 3). The XPS survey and atomic percentage of each element for both GO and Ca-GOA are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ef. Around 5.1% of calcium atoms are detected on the Ca-GOA surface, while in the original GO, no calcium was detected. This confirms the successful intercalation of Ca-ions into the Ca-GOA samples. It is noted that a small number of chlorine and sulfur atoms are also detected on Ca-GOA however are not the main concern in this study.\u003c/p\u003e\n\u003cp\u003eTo investigate the hydrogen-bond network in the Ca-GOA, we performed AWC measurements as described in the \u003cspan class=\"InternalRef\"\u003emethods\u003c/span\u003e section and compared the results to literature values from our own studies and others\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea-c show the recorded and reported adsorption isotherms. The water uptake (m/m\u003csub\u003e0\u003c/sub\u003e) of Ca-GOA was recorded with varying relative humidity (RH%) at constant room temperature (298K). The relative humidity is then converted into the partial pressure of water vapor to visualise the isotherm. The results show that AWC ability of Ca-GOA samples is significantly improved to up to 2.1 g/g from pure GO samples (GOM) with 0.5 g/g, which indicates the enriched hydrogen network on Ca-GOA surface. It should be noted that the referred GO adsorption curve was measured for GO membranes (GOM) and not in the form of aerogel. However, we ruled out the influence of materials morphology for AWC by showing that GOM and pure GOA have similar AWC ability (supplementary note 3).\u003c/p\u003e\n\u003cp\u003eFurther analysis of the isotherms allows us to get a deeper understanding of interfacial water molecules during the AWC. All three different materials, CaCl\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e35\u003c/sup\u003e, GOM\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e and our measured data of Ca-GOA, fit well under the BET model\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. However, the fitted BET equations are notably different from each (see supplementary note 4 for fitting details). The degree of the polynomial function of the BET equation, represents the number of adsorbed water molecule layers (\u003cem\u003en\u003c/em\u003e) on the surface of the material\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. Here, n\u0026thinsp;=\u0026thinsp;7 for CaCl\u003csub\u003e2\u003c/sub\u003e, n\u0026thinsp;=\u0026thinsp;4 for GOM and n\u0026thinsp;=\u0026thinsp;6 for Ca-GOA. Hence, the intercalation of Ca into Ca-GOA results in a stronger hydrogen network on surface compared to pure GO. In other words, the intercalation of cation enhances the interfacial water hydrogen network at the oxidized carbon surface, however the mechanism of such enhancement is still unclear.\u003c/p\u003e\n\u003cp\u003eBased on the BET model fitting analysis, we established that the Ca-intercalation offers strong adsorption sites for water molecules on the Ca-GOA surface. If that is the case, the adsorption isotherm should follow the Freundlich model, as it represents a heterogeneous surface adsorption process\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. In particular, Freundlich model describes a material with special adsorbing sites which are heterogeneously distributed on the adsorbent surface\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. In this case, the Ca atoms are the strong water-attracting adsorbing sites, well-dispersed on the GO plane, allowing coverage of the whole surface with several layers of water molecules\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed-f, the water adsorption isotherm of Ca-GOA can be fitted in high agreement with the typical Freundlich isotherm model, while being less in agreement with GO and CaCl\u003csub\u003e2\u003c/sub\u003e. The detailed information for Freundlich model fitting is shown in supplementary note 5.\u003c/p\u003e\n\u003cp\u003eThe trend of water molecules per oxygen and calcium atom of GOM, GOA, Ca-GOA and CaCl\u003csub\u003e2\u003c/sub\u003e versus the water partial pressure are shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eg-h. The uptake of water molecules per oxygen atom on Ca-GOA is up to 8 times higher than that of GOM and GOA at all ambient environments. Surprisingly, the water uptake per calcium ion, is up to 2.7 times higher in Ca-GOA compared to CaCl\u003csub\u003e2\u003c/sub\u003e at the highest water partial pressure (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ei).\u003c/p\u003e\n\u003cp\u003eHere, we can conclude that the water uptake per calcium and oxygen atom in Ca-GOA is much higher than in the individual materials, CaCl\u003csub\u003e2\u003c/sub\u003e and GOA/GOM. Keeping in mind that the Ca-sites were identified as the main contributor to the water uptake in Ca-GOA, it is striking to see that the water uptake per calcium is 2.7 times higher in the Ca-GOA compared to CaCl\u003csub\u003e2\u003c/sub\u003e. This strongly suggests that this enhancement is linked to an interplay between the functional groups of GO and the intercalated cations to form a synergistically enhanced hydrogen-bond network. To investigate this hypothesis, we further performed density functional theory (DFT) calculations.\u003c/p\u003e\n\u003cp\u003eWe examine the hydrogen bond properties between oxygen functionalities and water molecules with and without the existence of calcium ions using DFT calculation. We select a graphene plane with a bare epoxide group as a simplified model of a GO nanoflake surface as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea. Epoxide and hydroxide groups are typical oxygen functionalities on the GO basal plane and have been experimentally confirmed to have a strong hydrogen bonding interaction with water molecules\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The DFT calculations were performed at the PBE0-D3BJ/def2-QZVPP level of theory\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb-c indicates the process of the epoxide hydrogen bonded to one and two water molecules. We obtain hydrogen bond distances and energies typical of moderate hydrogen bonds\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. Namely, both water molecules hydrogen bonded to the epoxide group with a distance range of 1.96\u0026ndash;1.99 \u0026Aring; and enthalpy of \u0026minus;\u0026thinsp;14.8 kJ/mol and \u0026minus;\u0026thinsp;13.4 kJ/mol for the first and second water molecule, respectively. However, when the hydrated calcium cation is bound to the epoxide group, the hydrogen bonding network presents significantly different properties. We note that when the epoxy oxygen is coordinated to the Ca cation, the oxygen is bound to the GO surface via one C\u0026ndash;O bond, as illustrated in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed-e. We note that the bonding situation in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed-e also represents the bonding of a hydrated Ca cation to a C\u0026ndash;O functional group on the GO surface\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. Hereinafter, the oxygen connected to the GO surface with a single C\u0026ndash;O bond will be referred to as the GO oxygen, rather than the epoxy oxygen.\u003c/p\u003e\n\u003cp\u003eWith the presence of calcium cation, our calculations suggest that the GO oxygen is coordinate to the hydrated calcium cation. This dramatically enhances the hydrogen-bond network surrounding GO oxygen. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed presents a scenario when one water molecule is hydrogen bonded to both the GO oxygen and the hydrated calcium ion. Compared with the system in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, this water molecule is now hydrogen bonded via three hydrogen bonds, one with the GO oxygen and two with the hydrating water molecules around the calcium ion. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed, the hydrogen bond formed with the GO oxygen (1.546 \u0026Aring;) is significantly shorter than the hydrogen bonds formed with the water molecules hydrating the calcium cation (1.699 and 1.725 \u0026Aring;). Furthermore, this hydrogen bond is also significantly shorter than that formed between the water molecule and bare epoxide group presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb. Similarly, in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee, we further show the scenario when the GO oxygen is hydrogen bonded to a second water molecule. The calculation shows that the length of the hydrogen bond between the GO oxygen and the second water molecule is also largely shortened, from 1.982 \u0026Aring; (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec) to 1.689 \u0026Aring; (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee).\u003c/p\u003e\n\u003cp\u003eFrom the hydrogen bond enthalpy point of view, our calculations show that the hydrogen bonding between the GO oxygen and water molecules are highly reinforced by the hydrated calcium ion. In the scenario of the bare epoxide group on the GO surface, the hydrogen bond enthalpy between the oxygen and water molecules is around \u0026minus;\u0026thinsp;14 kJ/mol, as mentioned above. However, in the presence of the hydrated calcium ion, the hydrogen bond enthalpy between the GO and water molecules in the system shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed increases to as much \u0026minus;\u0026thinsp;66.7 kJ/mol and in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ee comes to \u0026minus;\u0026thinsp;52.9 kJ/mol. Thus, the H-bond enthalpy in the presence of the hydrated calcium ion is about 3\u0026ndash;4 times higher than that of a bare system. Both the increased hydrogen bond enthalpy and shorter hydrogen bond lengths are attributed to both the larger hydrogen bond network and stronger hydrogen bonding acceptor strength of the calcium ion decorated GO oxygen.\u003c/p\u003e\n\u003cp\u003eWe further investigate the hydrogen bonding acceptor properties of the GO oxygen based on the atomic polar tensor (APT) charges\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. The results are shown in Table\u0026nbsp;1. We obtain the following APT charges of the epoxide oxygen, \u0026minus;\u0026thinsp;0.40 \u003cem\u003ee\u003c/em\u003e, \u0026minus;\u0026thinsp;0.47 \u003cem\u003ee\u003c/em\u003e, and \u0026minus;\u0026thinsp;0.51 \u003cem\u003ee\u003c/em\u003e in the system shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea-c, respectively. As expected, the negative charge on the oxygen increases with the number of hydrogen bonds it is involved in. The charge on the hydrogen and oxygen atoms of the water molecules remains relatively constant for systems 3b and 3c. Namely, they range between +\u0026thinsp;0.23 \u003cem\u003ee\u003c/em\u003e and +\u0026thinsp;0.27 \u003cem\u003ee\u003c/em\u003e for the hydrogen and between \u0026minus;\u0026thinsp;0.45 \u003cem\u003ee\u003c/em\u003e and \u0026minus;\u0026thinsp;0.49 \u003cem\u003ee\u003c/em\u003e for the oxygen. Coordination of the hydrated calcium cation to the GO oxygen results in a dramatic change in the oxygen charge. In particular, the coordination of the hydrated calcium cation increases the negative charge on the GO oxygen from \u0026minus;\u0026thinsp;0.40 \u003cem\u003ee\u003c/em\u003e to as much as \u0026minus;\u0026thinsp;0.86 \u003cem\u003ee\u003c/em\u003e. This significant increase in negative charge on the GO oxygen makes it a much stronger hydrogen bond acceptor. Accordingly, the H-bond distance with the water molecule is shortened from 1.965 \u0026Aring; (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb) to 1.546 \u0026Aring; (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed). We note that the later hydrogen bond distance represents an exceptionally short hydrogen bond for a HOH\u0026bull;\u0026bull;\u0026bull;O system (i.e., a water molecule coordinated to an oxygen atom)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. It also reveals that coordination of calcium ion to the GO oxygen alters the atomic charges on the hydrogen and oxygen of the hydrogen-bonded water molecules. For example, for the systems depicted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb and \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed, the positive charge on the hydrogen increases from +\u0026thinsp;0.27 \u003cem\u003ee\u003c/em\u003e to +\u0026thinsp;0.37 \u003cem\u003ee\u003c/em\u003e and the negative charge on the oxygen increases from \u0026minus;\u0026thinsp;0.49 \u003cem\u003ee\u003c/em\u003e to \u0026minus;\u0026thinsp;0.63 \u003cem\u003ee\u003c/em\u003e (see Table\u0026nbsp;1). Remarkably, this demonstrates significant medium-range effects of the hydrated calcium cation on the charge of an oxygen centre to which it is bound via a hydrogen bond network (i.e., not covalently bound). The above results indicate that the hydrogen bonding ability of one epoxide group on the GO surface is enhanced by the coordination of the hydrated calcium ion. Such enhancement was shown via the increasing hydrogen bond acceptor strength of GO oxygen and additional hydrogen bonding interactions with the first hydration sphere of calcium cation. It is evident, both theoretically and experimentally, that epoxide groups on a GO surface tend to cluster in islands rather than be uniformly distributed across the surface\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. This leads to a natural question, whether this single hydrated calcium cation can interact with more than one epoxide group on GO surface.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Taba\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cp\u003eTable 1\u003c/p\u003e\n\u003cp\u003eAtomic polar tensor (APT) charge (\u003cstrong\u003eq\u003c/strong\u003e) in a.u. involved in the hydrogen bonds and on the Ca atom for the systems in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eModel\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eq\u003c/em\u003e (O\u003csub\u003ewater\u003c/sub\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eq\u003c/em\u003e (H\u003csub\u003ewater\u003c/sub\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eq\u003c/em\u003e (O\u003csub\u003eepoxy\u003c/sub\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eq\u003c/em\u003e (Ca)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3a\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026ndash;0.40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3b\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;0.49\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;0.27\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026ndash;0.47\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3c\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;0.48, \u0026minus;\u0026thinsp;0.45\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;0.26, +\u0026thinsp;0.23\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026ndash;0.51\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3d\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;0.63\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;0.37\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026ndash;0.84\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;1.34\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;0.61, \u0026minus;\u0026thinsp;0.60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;0.40, +\u0026thinsp;0.35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e\u0026ndash;0.84\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e+\u0026thinsp;1.25\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003csup\u003e*\u003c/sup\u003e N/A corresponds to the absence of atomic polar tensor charge.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eComparison between the hydrogen binding enthalpies at 298 K (\u003cstrong\u003e∆\u003c/strong\u003e\u003cstrong\u003eH\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e298,bind\u003c/strong\u003e\u003c/sub\u003e, in kJ/mol) for the systems in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e obtained in the solid state and in bulk aqueous solution.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth rowspan=\"2\" align=\"left\"\u003e\n\u003cp\u003eModel\u003c/p\u003e\n\u003c/th\u003e\n\u003cth colspan=\"2\" align=\"left\"\u003e\n\u003cp\u003e∆H\u003csub\u003e\u003cem\u003e298, bind\u003c/em\u003e\u003c/sub\u003e (kJ/mol)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSolid state\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAqueous solution\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3a\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3b\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;14.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;6.8\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3c\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;13.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;5.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3d\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;66.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;42.7\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;52.9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u0026ndash;32.9\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003csup\u003e*\u003c/sup\u003e N/A corresponds to the absence of hydrogen binding enthalpy.\u003c/p\u003e\n\u003cp\u003eWe further investigate the system with two epoxides on a graphene plane coordinated to a single hydrated calcium ion (see supplementary note 7). Each of the two oxygens on the GO surface is able to hydrogen bond to two water molecules. For the first three water molecules, we obtain binding enthalpies that are similar to those obtained from the functionalized systems in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e, namely 70.2, 58.5, and 63.6 kJ/mol, respectively. For the fourth water molecule, we obtain a lower binding enthalpy of 29.9 kJ/mol; this reduction is partly attributed to two (rather than three) hydrogen bonds in which this water molecule is involved in (supplementary note 7). Importantly, all these binding enthalpies are significantly larger than those obtained from the systems with the absence of hydrated calcium cation (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb-c).\u003c/p\u003e\n\u003cp\u003eWith the results above, we can now further optimize the hydrogen bond enthalpies from the perspective of the experimental conditions. This is because the calcium intercalated GO surface in the experimental settings is expected to be intermediate between the solid state and a bulk aqueous solution. It is well-established that hydrogen bond strengths are influenced by the effect of the solvent. In particular, the H-bond strength with the GO surface decreases with the polarity of the medium in the order of solid-state\u0026thinsp;\u0026gt;\u0026thinsp;non-polar solvents\u0026thinsp;\u0026gt;\u0026thinsp;polar solvents. Thus, the calculated hydrogen bond enthalpies above, which do not include solvent corrections, are expected to represent the upper limits for the experimental setting. Therefore, it is instructive to calculate the hydrogen bond enthalpies in bulk aqueous solution to obtain lower limits for the hydrogen-bond enthalpies. For this purpose, we use the conductor-like polarizable continuum model (CPCM)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e, which has been found to provide good performance for aqueous solution\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e58\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e. The solvation corrections reduce the hydrogen bond enthalpies for the unfunctionalized GO models by a factor of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\sim\\)\u003c/span\u003e\u003c/span\u003e2.5, whereas they reduce the bond enthalpies for the Ca-functionalized GO models by a factor of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\sim\\)\u003c/span\u003e\u003c/span\u003e1.5. As shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, the inclusion of the solvation corrections widens the gap between the hydrogen bond enthalpies of the GO models with and without coordination of the calcium ion. Considering that the GO surface in the experimental settings is expected to be intermediate between the solid state and a bulk aqueous solution, this is strong evidence to explain our experimental observation.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eHere, we uncover the synergistic hydrogen-bond network of functionalised carbon in the presence of a hydrated cation. Both, our experimental and computational results show a strong increase in hydrogen bond strength in the system of an epoxy functional group in close range to a hydrated calcium ion on graphene plane. Via AWC experiments we observed that the water uptake per calcium is dramatically increased, by up to a factor of 2.7 times higher in Ca-GOA compared to pure CaCl\u003csub\u003e2\u003c/sub\u003e. Similarly, the water uptake per oxygen of GOA is dramatically increased by a factor of 8 after intercalation of Ca-ions.\u003c/p\u003e \u003cp\u003eVia extensive DFT calculations, we uncover that the system of hydrated calcium ion and epoxy functional group on graphene plane enhances the overall binding strength of the hydrogen bond network. Particularly, the calcium ion increases the charge polarisation of the oxygen and hydrogen atoms of the C-O bond and water molecules. This leads to a higher enthalpy and shorter hydrogen bond lengths explaining the experimentally observed enhancement of water uptake per calcium ion.\u003c/p\u003e \u003cp\u003eThis study holds significance for numerous systems where hydrated ions are in proximity to carbon-based functional groups. As these are omnipresent in nature and technology, our study may help to bring a new perspective to a wide range of natural phenomena and technology applications that involve our described model system. Remarkably, we show that the AWC ability of GO can be enhanced to similar levels as pure CaCl\u003csub\u003e2\u003c/sub\u003e in terms of water uptake per gram. This may open new exciting opportunities to utilise Ca-GOA as a powerful desiccant in atmospheric water generation and energy efficient dehumidification.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cb\u003eMaterials\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe GO solution (15 mg/ml) were prepared via Hummer\u0026rsquo;s method and were supplied by NiSiNa Materials Japan. Calcium chloride (CaCl\u003csub\u003e2\u003c/sub\u003e) dihydrate powder was purchased from Sigma Aldrich.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of aerogels\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe Ca-GO solution was prepared by mixing predetermined volume of CaCl\u003csub\u003e2\u003c/sub\u003e salt solution and GO solution, followed by magnetic stirring for 30 mins at 1000 rpm. The as prepared solution was then freeze dried using a vacuum freeze drier at -60 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(℃\\)\u003c/span\u003e\u003c/span\u003e to synthesize Ca-GOAs. The GOA samples were prepared by freeze drying the GO solution without further modification. All prepared aerogel samples were stored in vacuum condition until experimental measurements. Synthesized samples are shown in supplementary note 1 (Fig.\u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e c-d).\u003c/p\u003e \u003cp\u003e \u003cb\u003eCharacterization of aerogels\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe X-ray photoelectron spectrometer (XPS, Thermo Scientific, UK ESCALAB250i) with mono-chromatic Al K alpha (energy 1486.68 eV) X-ray source was conducted to investigate the chemical composition of the synthesized aerogels. The C1s peak (284.5 eV) of graphite was used as a reference. The surface morphologies of the aerogels were visualized using a field-emission scanning electron microscope (FEI Nova NanoSEM 230 FE-SEM) with an energy dispersive spectroscopy (EDS) detector (Bruker SDD-EDS) for mapping the qualitative elemental composition.\u003c/p\u003e \u003cp\u003e \u003cb\u003eAtmospheric water capture tests\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe atmospheric water capture (AWC) tests of GOAs and Ca-GOAs were caried out in a custom-designed humidity controlling system (see supplementary note 8). The weight changes of the aerogels were continuously recorded every 10 seconds using a computer-controlled mass balance during the AWC process. The humid environment was maintained stable overnight (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{R}\\text{H}\\pm 3\\%\\)\u003c/span\u003e\u003c/span\u003e) before AWC tests. Experiments were repeated more than 2 times to study the standard deviation.\u003c/p\u003e \u003cp\u003e \u003cb\u003eComputational details\u003c/b\u003e \u003c/p\u003e \u003cp\u003eDensity functional theory (DFT) calculations were performed to gain further insights into the experimental findings using the Gaussian 16 software\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e. All geometries were fully optimized using the PBE0 exchange-correlation functional\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e in conjunction with def2-SVP basis set\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e (see supplementary note 9). Empirical dispersion corrections\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e are included using the Becke\u0026ndash;Johnson potential (denoted by the suffix D3BJ)\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e. Zero-point vibrational energy and enthalpic temperature corrections have been obtained from these calculations. The equilibrium structures were verified to have all real harmonic frequencies, confirming they are local minima on the potential-energy surface. The final electronic energies were refined using the PBE0-D3BJ functional in conjunction with the much larger quadruple-ζ def2-QZVPP basis set\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. Bulk solvent effects in aqueous solution were also considered using the conductor-like polarizable continuum model (CPCM) continuum solvation model\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the funding for supporting this project from Vesi Water Pty Ltd. X.R. acknowledges the UNSW UIPA Scholarship. The authors acknowledge the staff from Mark Wainwright Analytical Centre at UNSW for technical assistance on sample characterizations using SEM and XPS. V.Q. acknowledges the funding from the European Union\u0026apos;s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie Grant Agreement No. 101066462. The present work was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. We gratefully acknowledge the system administration support provided by the Faculty of Science, Agriculture, Business and Law at the University of New England to the Linux cluster of the Karton group. D.V.A. and K.S.N. are supported by the Ministry of Education, Singapore, under its Research Centre of Excellence award to the Institute for Functional Intelligent Materials (I-FIM, project no. EDUNC-33-18-279-V12). Xiao Sui conducted this work when she was a research scientist in UNSW.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eX.R., X.S., T.F., D.J., L.O., and R.J. are co-inventors of a PCT application (No. PCT/AU2022/051080). The authors declare no other competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eR.J. conceived the idea and developed the project with L.O. X.R., and X.S. conducted the experiments. X.R. analysed the data with T.F., R.J., and L.O. A. K. led the computational simulations and contributed to the analysis of results and to develop the mechanism. X.R. and T.F. wrote the first draft and R.J. revised the draft. D.J., X.W., Y.N., K.K.P., V.Q., D.V.A. and K.S.N. contributed to the analysis, discussions and also critically review/edit the draft. R.J. supervised the project.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eVerdaguer, A., Sacha, G. M., Bluhm, H. \u0026amp; Salmeron, M. 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Effect of the damping function in dispersion corrected density functional theory. \u003cem\u003eJ Comput Chem\u003c/em\u003e \u003cstrong\u003e32\u003c/strong\u003e, 1456\u0026ndash;1465 (2011).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4010517/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4010517/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWater molecules at the solid-liquid interface display intricate behaviours sensitive to small changes. The presence of different interfacial components, such as cations or functional groups, shape the physical and chemical properties of the hydrogen bond network. Understanding such interfacial hydrogen-bond networks is essential for a large range of applications and scientific questions. To probe the interfacial hydrogen-bond network, atmospheric water capture is a powerful tool. Here, we experimentally observe that a calcium ion on a calcium-intercalated graphene oxide aerogel (Ca-GOA) surface captures 2.7 times more water molecules than in its freestanding state. From density functional theory (DFT) calculations, we uncover the synergistically enhanced hydrogen-bond network of the calcium ion-epoxide complex due to significantly larger polarizations and hydrogen bond enthalpies. This study reveals valuable insights into the interfacial water hydrogen-bond network on functionalized carbon-cation complexed surfaces and potential pathways for future atmospheric water generation technologies.\u003c/p\u003e","manuscriptTitle":"Synergetic hydrogen-bond network of functionalized graphene and cations for enhanced atmospheric water capture","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-21 15:38:45","doi":"10.21203/rs.3.rs-4010517/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"887b5875-2776-4ce3-9007-c9f62ab785dd","owner":[],"postedDate":"March 21st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":29679768,"name":"Physical sciences/Materials science/Nanoscale materials/Graphene"},{"id":29679769,"name":"Physical sciences/Chemistry/Materials chemistry/Graphene"}],"tags":[],"updatedAt":"2024-09-19T16:15:23+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-21 15:38:45","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4010517","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4010517","identity":"rs-4010517","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}