⇦Chinese Physics Letters, 2021, Vol. 38, No. 10, Article code 106801 Effect of Oxide Content of Graphene Oxide Membrane on Remarkable Adsorption for Calcium Ions Jie Jiang (江杰)1,2, Long Yan (闫隆)1*, and Haiping Fang (方海平)1,3 Affiliations 1Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China 2University of Chinese Academy of Sciences, Beijing 100049, China 3Department of Physics, East China University of Science and Technology, Shanghai 200237, China Received 6 August 2021; accepted 14 September 2021; published online 28 September 2021 Supported by the National Natural Science Foundation of China (Grant No. 11675246).
*Corresponding author. Email: yanlong@sinap.ac.cn Citation Text: Jiang J, Yan L, and Fang H P 2021 Chin. Phys. Lett. 38 106801    Abstract Graphene oxide membranes (GOMs), as one of the most promising novel materials, have gained great interest in the field of adsorption. However, the oxygen content of graphene oxide is directly related to its adsorption properties, such as suspension stability, adsorption capacity, and reusability of GOMs. Here, a series of reduced GOMs with oxygen content from 28% to 12% were conveniently prepared by the thermally reduced and the corresponding interlayer spacing of these membranes changed from 8.0 Å to 3.7 Å. These prepared GOMs have remarkable Ca$^{2+}$ adsorption capacity, which increases with the oxygen content or interlayer spacing of GOMs. Importantly, the max adsorption capacity of the mass ratio between adsorbed Ca$^{2+}$ and pristine GOMs can reach up to 0.481 g/g, which is about one order of magnitude higher than the adsorption capacity of activated sludge, magnetic Fe$_{3}$O$_{4}$, functionalized silica, zeolite molecular sieve, and other reported previously. Moreover, GOMs show excellent stability and the Ca$^{2+}$ can be easily desorbed by water, so that the GOMs can be reused. Our previous theoretical analysis suggests that this remarkable adsorption is attributable to the strong interactions between Ca$^{2+}$ and GO sheets, including the ion-$\pi$ interactions between Ca$^{2+}$ and aromatic graphitic rings as well as the electrostatic interaction between Ca$^{2+}$ and oxygen-containing groups. DOI:10.1088/0256-307X/38/10/106801 © 2021 Chinese Physics Society Article Text Graphene oxide is a two-dimensional (2D) material derived from the oxidation of graphite, has attracted great interests due to their unique physical and chemical characteristics.[1–4] They show potential applications in many diverse technology fields, such as ion sieves, energy storage, water treatment and adsorption.[5–10] The performances of these applications are closely correlated to the characteristics of GO, such as its conductivity, water stability, specific surface area and adsorption performance.[10–14] Cation-$\pi$ interactions, characterized by the adsorption between various cations and $\pi$ electron of aromatic rings, play a dominant role in physics, biology, chemistry and materials sciences. Hence, cation-$\pi$ interactions are not negligible under the systems composed of cations and aromatic rings. Moreover, graphene oxide (GO) is rich in oxygen-containing groups, and the electrostatic interaction between ions and oxygen-containing groups also has influence on adsorption of GO.[15] The adsorption performance of GOMs should be collectively affected by their specific surface, aromatic ring structures and oxygen-containing groups. Therefore, it is important and necessary to further systematically study the factors affecting the adsorption performance of GOMs. Moreover, the adsorption of calcium ions is of great significance in water purification and treatment. In this work, we prepared a series of reduced GOMs with different oxygen contents by thermal reduction method.[16] Remarkable amount of Ca$^{2+}$ can be adsorbed inside these GOMs by using isothermal adsorption experiments. The adsorption capacity of Ca$^{2+}$ inside the GOMs with highest oxygen content of 28% can reach as high as 0.481 g/g. Remarkably, the GOMs exhibited superior adsorption performance compared to other adsorbents, as the adsorption capacity of activated sludge, magnetic Fe$_{3}$O$_{4}$, functionalized silica and zeolite molecular sieve for Ca$^{2+}$ were 0.019 g/g, 0.033 g/g, 0.041 g/g, and 0.052 g/g, respectively.[17,18] Characterization of these GOMs contained CaCl$_{2}$ (CaCl$_{2}$@GOMs) by using scanning electron microscopy (SEM) further confirmed the extensive distribution of CaCl$_{2}$ inside. Our x-ray photoelectron spectroscopy (XPS) and XRD results showed that the Ca$^{2+}$ adsorption capacity of GOMs increases with the oxygen content or interlayer spacing of GOMs. It is also noted that the ions inside GOMs can be easily desorbed by water so that the GOMs can be reused. Our findings reveal the remarkable adsorption capacity inside the GOMs and the underlying mechanism based on the strong interactions between ions and GO sheets, including ion-$\pi$ interactions between ions and aromatic graphitic rings as well as the electrostatic interaction between cation and oxide groups. These have potential applications in the field of water purification, ion separation, capacitor and battery. Freestanding GOMs (GOMs)[19–25] were prepared from the GO suspension via the drop-casting method. These GOMs were heat treated at 60 ℃, 90 ℃, 120 ℃, 150 ℃ and 180 ℃ for one hour to obtain reduced GOMs with different oxygen contents. These membranes were then immersed in CaCl$_{2}$ solutions with concentrations of 1.7 mol/L, 3.4 mol/L, 5.0 mol/L, 6.7 mol/L to obtain the corresponding CaCl$_{2}$@GOMs.
cpl-38-10-106801-fig1.png
Fig. 1. Case of CaCl$_{2}$@GOMs. (a) Synthesis procedure of CaCl$_{2}$@GOMs. (b) Adsorption capacity of Ca$^{2+}$ in CaCl$_{2}$@GOMs with different reduction temperatures. (c) Adsorption capacity of Ca$^{2+}$ in CaCl$_{2}$@GOMs after immersing in CaCl$_{2}$ solution with various concentrations of 1.7 mol/L, 3.4 mol/L, 5.0 mol/L, and 6.7 mol/L. Error bars indicate the standard deviation.
We first study the CaCl$_{2}$ adsorption in the CaCl$_{2}$@GOMs with different reduction temperatures and define the adsorption capacity as the mass ratio between adsorbed Ca$^{2+}$ and pristine GOMs. These membranes were immersed in the CaCl$_{2}$ solutions with concentrations of 6.7 mol/L. The metal ions in the CaCl$_{2}$@GOMs were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) after removal of the solution on the surface. Then, we obtained the adsorption capacity for GOMs with different reduction temperature. Figure 1(b) shows that the adsorption capacity of CaCl$_{2}$@GOMs is 0.481 g/g, 0.437 g/g, 0.266 g/g, 0.203 g/g, 0.098 g/g and the corresponding reduction temperature is 60 ℃, 90 ℃, 120 ℃, 150 ℃, and 180 ℃, respectively. Furthermore, the membranes reduced in 60 ℃ were then immersed in the CaCl$_{2}$ solution with concentrations of 1.7 mol/L, 3.4 mol/L, 5.0 mol/L, and 6.7 mol/L. The adsorption capacity of Ca$^{2+}$ in the CaCl$_{2}$@GOMs increases from $\sim $0.058 g/g to $\sim $0.481 g/g with the concentration of CaCl$_{2}$ solution from quarter saturation to saturation [Fig. 1(c)]. Thus, the GOMs heat-treated at 60 ℃ have the max adsorption capacity $\sim $0.481 g/g. Remarkably, the GOMs exhibited superior adsorption performance compared to other adsorbents, as the adsorption capacity of activated sludge, magnetic Fe$_{3}$O$_{4}$, functionalized silica and zeolite molecular sieve for Ca$^{2+}$ were 0.019 g/g, 0.033 g/g, 0.041 g/g, and 0.052 g/g, respectively [Fig. 2(a)]. The max adsorption capacity of different materials was obtained from the isothermal adsorbing experiment, and the adsorption isotherm always used to describe the quantity of adsorbate on the surface as a function of its concentration at constant temperature.
cpl-38-10-106801-fig2.png
Fig. 2. (a) Adsorption capacity of different adsorbents for Ca$^{2+}$. (b) Desorption process of CaCl$_{2}$ from CaCl$_{2}$@GOMs after immersing in DI water of 50 mL. The process can be divided into three stages: (I) fast desorb stage (cyan-blue region), (II) slow desorb stage (purple region), (III) stable stage (yellow region). (c) Full-scan x-ray photoelectron spectrometer spectra of the samples of GOMs reduced in different temperatures. (d) Elemental atomic content (at.%) of C and O with respect to the reduction temperature.
While those CaCl$_{2}$@GOMs with the adsorption capacity of $\sim $0.481 g/g are immersed into deionized (DI) water, CaCl$_{2}$ can be easily desorbed. Figure 2(b) shows the concentrations of CaCl$_{2}$ after immersing in DI water. The concentration of CaCl$_{2}$ in the solution increases rapidly from 0 to 50 ppm in the first 1 min (stage I), and then slowly to 75 ppm in the range of 1–5 min (stage II). Afterwards, the concentration of CaCl$_{2}$ becomes stable (stage III). This indicates that the desorption process of CaCl$_{2}$ from the CaCl$_{2}$@GOMs has finished. Thus, GOMs show excellent stability in salt solution and can be easily desorbed by water, so that the GOMs can be reused. XPS was utilized to analyze the oxygen group content. The XPS full scan spectra and C $1s$ and O $1s$ partial spectra of GOMs under different reduction temperatures are shown in Fig. 2(c). The C $1s$ and O $1s$ peak positions of reduced GOMs were at 285.08 eV and 532.08 eV, respectively. From the elemental contents of C and O in the full scan spectra, it is obvious that the oxygen content in each sample decreased sharply (from 23.2% to 12.1%) with increasing the reduction temperature from 60 ℃ to 180 ℃ [Fig. 2(d)], indicating the successful removal of oxygen-containing groups on the GO surface. Then, we used x-ray diffraction to analyze the reduced GOMs obtained with the different reduction temperatures. The interlayer spacings (denoted as $d$) of the reduced GOMs were indicated by the Bragg peaks in the XRD spectrum. There are peaks at 11.0$^{\circ}$, 11.6$^{\circ}$, 12.6$^{\circ}$, 24.3$^{\circ}$, and 24.3$^{\circ}$, indicating that $d$ was 8.0 Å, 7.6 Å, 7.0 Å, 3.7 Å, and 3.7 Å for the GOMs reduced at 60 ℃, 90 ℃, 120 ℃, 150 ℃, 180 ℃ membranes, respectively, as shown in Fig. 3(a). As the reduction temperature increased, the interlayer spacing of reduced GOMs decreased from 8.0 Å to 3.7 Å. This means that the interlayer spacing of GOMs is closely related to the oxygen content.
cpl-38-10-106801-fig3.png
Fig. 3. (a) XRD patterns of the GOMs reduced at various temperatures. (b) Cross-sectional scanning electron microscope image. (c) Elemental mappings of CaCl$_{2}$@GOMs based on the SEM analysis. The elements of C, O, Ca, Cl are displayed by the colors of cyan, red, blue, and green, respectively.
Figure 3(b) shows a cross-sectional view of GOMs and Fig. 3(c) shows the SEM image of the CaCl$_{2}$ distribution inside the CaCl$_{2}$@GOMs with the adsorption capacity of $\sim $0.481 g/g. The elemental mappings of C, O, Ca, Cl elements are shown. We find that the signals of Ca and Cl elements in the region covered with large amount of CaCl$_{2}$ are shown, in much higher intensities than the intensities of C and O elements, which suggests remarkable amount of Ca and Cl elements. Conventional porous materials without aromatic rings, such as zeolite or silica, can adsorb CaCl$_{2}$ for desalting or purification based on the van der Waals interaction and ion exchange mechanism. The adsorption capacity of these materials is low, resulting to the limited efficiency for practical applications. However, in our experiment, we note that the adsorption capacity of CaCl$_{2}$ in CaCl2@GOMs is remarkable, much larger than those in other porous materials. Different from the simple adsorption in other porous materials due to the mechanism of van der Waals interaction or ion exchange, we attribute this remarkable adsorption of CaCl$_{2}$ in our CaCl$_{2}$@GOMs to the strong interactions between CaCl$_{2}$ and GO sheets, including the ion-$\pi$ interactions between Ca$^{2+}$/Cl$^{-}$ and the aromatic graphitic rings as well as the electrostatic interaction between Ca$^{2+}$ and oxide groups. In summary, we have synthesized a series of reduced GOMs with different oxygen contents from 28% to 12% and the corresponding interlayer spacings of these membranes changed from 8.0 Å to 3.7 Å. Then, we have studied the effect of oxygen content and initial salt solution concentration on the adsorption performance of GOMs. The adsorption performance of CaCl$_{2}$/GOMs shows that the best adsorption effect occurs when the reduction temperature is 60 ℃, which stands for the maximum equilibrium adsorption capacity (0.481 g/g). Moreover, we find that GOMs show excellent stability in salt solution and can be easily desorbed by water, so that the GOMs can be reused. These results are further demonstrated by our XRD pattern and confirmed by SEM experiments, and similar results are found in the adsorption process of NaCl as well (see Fig. S1 in the Supplementary Material). The key to this observation is the strong interactions between ions and GO sheets, including the ion-$\pi$ interactions between ions and aromatic graphitic rings as well as the electrostatic interaction between ions and oxide groups. Our findings reveal the remarkable adsorption. Also, we have studied the effect of oxygen content and initial salt solution concentration on the adsorption performance of GOMs, which has potential applications in the fields of water purification, ion separation, capacitor and battery. References The chemistry of graphene oxideGraphene and Graphene Oxide: Synthesis, Properties, and ApplicationsThe reduction of graphene oxideGraphene Oxide-Based Q-Switched Erbium-Doped Fiber LaserTunable sieving of ions using graphene oxide membranesPrecise and Ultrafast Molecular Sieving Through Graphene Oxide MembranesHierarchical Nanocomposites of Polyaniline Nanowire Arrays on Graphene Oxide Sheets with Synergistic Effect for Energy StorageRecent advances in the efficient reduction of graphene oxide and its application as energy storage electrode materialsApplication of response surface methodology to optimize the adsorption performance of a magnetic graphene oxide nanocomposite adsorbent for removal of methadone from the environmentTemperature-dependent properties of metastable graphene oxideGraphene oxide coated quartz sand as a high performance adsorption material in the application of water treatmentPreparation and application of reduced graphene oxide as the conductive material for capacitive deionizationReinforced natural rubber nanocomposites using graphene oxide as a reinforcing agent and their in situ reduction into highly conductive materialsGraphene Oxide Membranes with Strong Stability in Aqueous Solutions and Controllable Lamellar SpacingAdsorption behaviour of reduced graphene oxide towards cationic and anionic dyes: Co-action of electrostatic and π – π interactionsCation–π Interactions in Graphene‐Containing Systems for Water Treatment and BeyondThe effect of degree of reduction on the electrical properties of functionalized graphene sheetsStudy on the adsorption of Ca2+, Cd2+ and Pb2+ by magnetic Fe3O4 yeast treated with EDTA dianhydrideControllable reduction of graphene oxide by electron-beam irradiationIon sieving in graphene oxide membranes via cationic control of interlayer spacingCorrectionThe rise of grapheneWater-Mediated Spontaneously Dynamic Oxygen Migration on Graphene Oxide with Structural Adaptivity for Biomolecule AdsorptionSuper-strong interactions between multivalent anions and graphene*Adsorption of NO2 by hydrazine hydrate-reduced graphene oxide
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