Fabrication and Characterization of Porous Core–Shell Graphene / SiO 2 Nanocomposites for the Removal of Cationic Neutral Red Dye

: Porous rGO / SiO 2 nanocomposites with a “core-shell” structure were prepared as an e ﬃ cient adsorbent for the liquid-phase adsorption of cationic neutral red (NR) dye. The samples were characterized with powder X-ray di ﬀ raction (XRD), ﬁeld-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and N 2 and water vapor adsorption / desorption methods. The NR removal ability and kinetics of the adsorption process of SiO 2 and the rGO / SiO 2 nanocomposites were investigated at 298 K. The rGO / SiO 2 nanocomposite SG 0.30 showed a superior adsorption of NR dye. In regard to NR at pH 5, we measured a superior adsorption capacity of 66.635 mg / g at an initial NR concentration of 50 mg / L. The experimental adsorption capacity of SG 0.30 was 3.791 times higher than that of SiO 2 . Then, we compared the results with similar materials used for NR removal. Moreover, the water adsorption sites provided by the nitrogen- and oxygen-containing groups might be one of the reasons for the increased adsorption of water vapor. The broad range of properties of the rGO / SiO 2 nanocomposite, including its simple synthesis, ability to be mass prepared, and strong adsorption properties, makes it a truly novel adsorbent that can be industrially produced, and shows potential application in the treatment of wastewater-containing dyes.


Introduction
With the development of modern industry, water pollution is one of the primary environmental issues in China because it can be harmful to human health and can affect people's lives [1,2]. Adsorption is an effective method of solving water pollution. Various materials, such as activated carbon [3,4], zeolites [5,6], metal oxides [7], and organic-inorganic hybrids [8], have been used as adsorbents. However, although adsorption with activated carbon is highly developed, the cost of reprocessing and regeneration is high. Additionally, the microporous structure of zeolites limits their application in the field of dye adsorption. Metal materials are costly and do not have good enough adsorption capacity. These limitations encourage the search for inexpensive and readily available materials as adsorbents for the removal of dyes.
Carbon materials have received attention from researchers because of their abundant sources, low cost, and stable performance. Graphene (G), known as a porous carbon material, has been increasingly studied as an adsorbent owing to its large surface area, layered structure, and efficient structure of graphene may effectively improve its potential for use in practical applications [13][14][15][16][17]. For example, Annamalai reported that graphene-loaded NiCo2S4 could effectively increase the capacitance of supercapacitors because of its porous structure [15]. Kim introduced mesopores in graphene via a template method in order to investigate its potential as an anode material for lithiumsulfur batteries [18]. Nanosilica has randomly packed pores and a large specific surface area, while also being hydrophilic [19][20][21]. The combination of nanosilica and graphene can effectively decrease the agglomeration of both materials. This composite is an inexpensive, renewable, and easily available adsorbent material with a high adsorption capacity, and shows promise for use in environmental and energy applications [22][23][24][25][26]. Wang et al. fabricated 3D-graphene aerogelsmesoporous silica materials-that exhibited a high absorption capacity for phenols because of their high surface area (1000.80 m 2 g −1 ), narrow mesopore size distribution (1.87 nm), and porous structures [27]. Gu et al. prepared sandwich-like multilayer silica-Fe3O4-GO composites with mesopores and a large surface area for the removal of methylene blue from water [28]. Li et al. synthesized Si/rGO nanosheet composites with a large surface area by a sol-gel process for the high-efficiency removal of methylene blue and thionine from water [29]. Thus, it was found that a high surface area, narrow mesopore distribution, and multilayer structure could improve the removal efficiency of contaminants in water. However, materials such as the silicon source tetraethyl orthosilicate (TEOS) greatly increase the cost of industrial applications. Therefore, finding a low-cost preparation method is a key issue for industrial production.
To find rGO/SiO2 composites that were low cost, could be prepared in large quantities, and had high adsorption capacity for removing water pollutants, we designed the following experiments. Herein, cationic neutral red (NR) dye (pH = 5) was selected as the model compound in order to evaluate the adsorption ability of rGO/SiO2 nanocomposites from an aqueous solution. The composites were synthesized by the post synthesis treatment illustrated in Scheme 1. By controlling the ratio of rGO and SiO2, the surface properties could be adjusted to improve the adsorption quantity of NR. The parameters were evaluated for NR removal, with a focus on the ratio of GO and SiO2, concentration of NR, adsorption quantity, equilibrium times, pseudo-first-order kinetic parameters, and pseudo-second-order kinetic parameters.  Deionized water was used for all of the experiments.

Characterizations
Field-emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS) of the samples were acquired on a JSM-6700 (JEOL) scanning electron microscope operating at 5 kV. Transmission electron microscopy (TEM) images of the samples were obtained on a Tecnai F20 (Philips) transmission electron microscope operating at 100 kV. Powder X-ray diffraction (XRD) patterns were obtained on a Miniflex 600 X-ray automatic diffractometer (Rigaku Corporation) with a Cu Kα (λ = 0.15406 nm) radiation source in an explored angular range of 10-65 degrees. Thermogravimetric analysis (TGA)/differential thermal analysis (DTA) was performed on a simultaneous TGA/DTA analyzer (NETZSCH STA449F3) from 30 to 800 • C at a heating rate of 10 • C min −1 in air. The nitrogen adsorption/desorption isotherms were measured at 77 K using an ASAP2020 (Micromeritics, Norcross, GA, USA) gas adsorption analyzer, and the samples were evacuated at 10 −4 Pa and 393 K for 2 h before measurement. The water vapor adsorption/desorption isotherms were tested at room temperature by an IGA100B (Hiden Isochema Ltd.) intelligent gravimetric sorption analyzer. The samples were evacuated at zero to a saturated vapor pressure and 393 K for 2 h before the measurement. The X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250Xi spectrometer using an Al Kα (1486.6 eV) radiation source to evaluate the chemical states of the elements on the surface of the samples. Raman spectra were obtained with a LabRAM HR system.

Experimental Method
The liquid phase adsorption of cationic neutral red (NR) dye (pH 5) on the sample was measured with a UV-1100 spectrophotometer (Shanghai Mapada Instruments Co., Ltd., Shanghai, China). Briefly, 20 mg of sample was added to 30 mL of a 10, 25, or 50 mg/L NR aqueous solution (pH 5). Adsorption was carried out with continuous stirring at 700 rpm at room temperature. At certain time intervals, 5 mL of the mixture was withdrawn and separated by centrifugation (5000 rad min −1 ). All of the test data were repeated 3 to 5 times and averaged, and the data error was controlled within 5%. The concentration of NR in the solution was determined at 530 nm. The adsorption capacity and removal efficiency of the samples were calculated by the following equations: where Q t (mg/g) is the amount of NR adsorbed on the adsorbent at time t, C 0 (mg/L) is the initial NR concentration, C t is the NR concentration at time t, V (L) is the volume of the solution, m (g) is the mass of the adsorbent, and E is the removal efficiency of the sample. Herein, to investigate the kinetics of the adsorption process, pseudo-first-order and pseudo-second-order adsorption models were chosen.
Pseudo-first-order equation [31]: Pseudo-second-order equation [32]: where Q e (mg/g) is the theoretical equilibrium saturated adsorption of NR, Q t (mg/g) is the amount of NR adsorbed on the adsorbent at time t, k 1 (g mg −1 min −1 ) is the pseudo-first-order rate constant of adsorption, and k 2 (g mg −1 min −1 ) is the pseudo-second-order rate constant of adsorption.

Characterizations of the Samples
The morphologies and nanostructures of the SiO 2 and rGO/SiO 2 nanocomposites were examined via FE-SEM and TEM. In Figure 1a,b, the primary SiO 2 was a microsphere with a diameter of approximately 10 nm. Nanosilica could effectively reduce agglomeration by using the surface-active agent PEG2000. In Figure 1d, the TEM image clearly shows that the rGO/SiO 2 composites had a spherical shell structure similar to a core-shell structure, and the inner layer of nanosilica was wrapped by the outer layer of graphene. A similar structure was observed in the SEM image in Figure 1c, where SiO 2 was wrapped in graphene powder with a rough surface. The distributions of the elements in the SiO 2 and rGO/SiO 2 nanocomposites were determined by EDS, as shown in Figure 1. In the rGO/SiO 2 composite, the weight percentages of C and Si were 11.09 and 37.33%, respectively. Figure 2 shows the TG/DTA of the SiO 2 and rGO/SiO 2 samples. In regard to SiO 2 , the thermal decomposition temperature was 479 • C. Free water and CO 2 in the pores or on the surface were first lost from 30 to 110 • C. Then, from 110 to 666 • C, the surface groups of silica gradually decomposed, and the reaction was exothermic. Next, from 666 to 720 • C, the reaction was exothermic, which might be because of the removal of the structural water by two Si-OH groups in the silica to form Si-O-Si bonds. Finally, from 720 to 800 • C, the silica continued to decompose. In regard to the rGO/SiO 2 sample, the thermal decomposition temperature was 436 • C. The free water and CO 2 in the pores or on the surface were lost from 30 to 100 • C, and the silica and rGO gradually decomposed from 100 to 800 • C.  Figure 2 shows the TG/DTA of the SiO2 and rGO/SiO2 samples. In regard to SiO2, the thermal decomposition temperature was 479 °C. Free water and CO2 in the pores or on the surface were first lost from 30 to 110 °C. Then, from 110 to 666 °C, the surface groups of silica gradually decomposed, and the reaction was exothermic. Next, from 666 to 720 °C, the reaction was exothermic, which might be because of the removal of the structural water by two Si-OH groups in the silica to form Si-O-Si bonds. Finally, from 720 to 800 °C, the silica continued to decompose. In regard to the rGO/SiO2 sample, the thermal decomposition temperature was 436 °C. The free water and CO2 in the pores or on the surface were lost from 30 to 100 °C, and the silica and rGO gradually decomposed from 100 to 800 °C. The powder XRD patterns of the rGO, SiO2, and rGO/SiO2 samples are shown in Figure 3. SiO2 exhibited characteristic diffraction peaks at 2θ values of 23°, and rGO showed characteristic diffraction peaks at 2θ values of 43°. In the rGO/SiO2 sample, the same peaks were shown at 2θ values of 23° and 43°, confirming that the rGO/SiO2 composites were prepared successfully.   Figure 2 shows the TG/DTA of the SiO2 and rGO/SiO2 samples. In regard to SiO2, the thermal decomposition temperature was 479 °C. Free water and CO2 in the pores or on the surface were first lost from 30 to 110 °C. Then, from 110 to 666 °C, the surface groups of silica gradually decomposed, and the reaction was exothermic. Next, from 666 to 720 °C, the reaction was exothermic, which might be because of the removal of the structural water by two Si-OH groups in the silica to form Si-O-Si bonds. Finally, from 720 to 800 °C, the silica continued to decompose. In regard to the rGO/SiO2 sample, the thermal decomposition temperature was 436 °C. The free water and CO2 in the pores or on the surface were lost from 30 to 100 °C, and the silica and rGO gradually decomposed from 100 to 800 °C. The powder XRD patterns of the rGO, SiO2, and rGO/SiO2 samples are shown in Figure 3. SiO2 exhibited characteristic diffraction peaks at 2θ values of 23°, and rGO showed characteristic diffraction peaks at 2θ values of 43°. In the rGO/SiO2 sample, the same peaks were shown at 2θ values of 23° and 43°, confirming that the rGO/SiO2 composites were prepared successfully. The nanopore structures were evaluated by nitrogen physical adsorption measurements at 77 K on the SiO 2 , GO, and rGO/SiO 2 samples, as shown in Figure 4. The Brunner−Emmet−Teller (BET) specific surface areas of SiO 2 and GO were 130 and 21 m 2 g −1 , respectively. After the composite was prepared, the BET specific surface area of the rGO/SiO 2 samples was 54 m 2 g −1 . The adsorption total pore volumes of SiO 2 , GO, and rGO/SiO 2 were 0.090, 0.016, and 0.113 cm 3 g −1 , respectively. The adsorption at P/P 0 < 0.1 suggested the presence of micropores, and the adsorption at P/P 0 >~0.4 (to~1) indicated the presence of stacked mesopores in the rGO/SiO 2 samples. Therefore, the rGO/SiO 2 sample had a porous structure with both micro-and meso-pores.  The nanopore structures were evaluated by nitrogen physical adsorption measurements at 77 K on the SiO2, GO, and rGO/SiO2 samples, as shown in Figure 4. The Brunner−Emmet−Teller (BET) specific surface areas of SiO2 and GO were 130 and 21 m 2 g −1 , respectively. After the composite was prepared, the BET specific surface area of the rGO/SiO2 samples was 54 m 2 g −1 . The adsorption total pore volumes of SiO2, GO, and rGO/SiO2 were 0.090, 0.016, and 0.113 cm 3 g −1 , respectively. The adsorption at P/P0 < 0.1 suggested the presence of micropores, and the adsorption at P/P0 > ~0.4 (to ~1) indicated the presence of stacked mesopores in the rGO/SiO2 samples. Therefore, the rGO/SiO2 sample had a porous structure with both micro-and meso-pores.  To characterize the bonding of the samples, XPS was used to study the functional groups on their surfaces. As shown in Figure 5a, the XPS spectra of GO, rGO, silica, SG 0.30, and SG 1.00 exhibited peaks at 531.9 (O 1s), 400.0 (N 1s), 284.4 (C 1s), and 102.8 eV (Si 2p), indicating the existence of silica on the surface. In Figure 5b, the XPS C 1s spectra of GO, rGO, SG 0.30, and SG 1.00 showed the presence of carbon functional groups, such as O-C=O, C-O-C, C-N, C-C, and C=C, in multiple chemical states and at various relative contents. To characterize the bonding of the samples, XPS was used to study the functional groups on their surfaces. As shown in Figure 5a, the XPS spectra of GO, rGO, silica, SG 0.30, and SG 1.00 exhibited peaks at 531.9 (O 1s), 400.0 (N 1s), 284.4 (C 1s), and 102.8 eV (Si 2p), indicating the existence of silica on the surface. In Figure 5b, the XPS C 1s spectra of GO, rGO, SG 0.30, and SG 1.00 showed the presence of carbon functional groups, such as O-C=O, C-O-C, C-N, C-C, and C=C, in multiple chemical states and at various relative contents. To characterize the bonding of the samples, XPS was used to study the functional groups on their surfaces. As shown in Figure 5a, the XPS spectra of GO, rGO, silica, SG 0.30, and SG 1.00 exhibited peaks at 531.9 (O 1s), 400.0 (N 1s), 284.4 (C 1s), and 102.8 eV (Si 2p), indicating the existence of silica on the surface. In Figure 5b, the XPS C 1s spectra of GO, rGO, SG 0.30, and SG 1.00 showed the presence of carbon functional groups, such as O-C=O, C-O-C, C-N, C-C, and C=C, in multiple chemical states and at various relative contents. The water adsorption isotherms of SiO2, SG 0.30, and SG 1.00 at 298 K are shown in Figure 6. As shown in Figure 5a, the water adsorption of SiO2 was nearly complete up to P/P0~0.5. Both SG 0.30 and SG 1.00 exhibited water adsorption up to P/P0~1.0, which was caused by some exposed surfaces of SiO2 and the graphene-formed micropores, as shown in Figure 6b,c. On the surface of SG 0.30 and SG 1.00, there were C-C, C-O-C, C=C, C-N, and O-C=O groups, providing adsorption sites for water. However, in SG 1.00, excess rGO coated the SiO2 and decreased the number of adsorption sites, which might be the reason why the adsorption isotherm differed from that of SG 0.30. The water adsorption The water adsorption isotherms of SiO 2 , SG 0.30, and SG 1.00 at 298 K are shown in Figure 6. As shown in Figure 5a, the water adsorption of SiO 2 was nearly complete up to P/P 0~0 .5. Both SG 0.30 and SG 1.00 exhibited water adsorption up to P/P 0~1 .0, which was caused by some exposed surfaces of SiO 2 and the graphene-formed micropores, as shown in Figure 6b,c. On the surface of SG 0.30 and SG 1.00, there were C-C, C-O-C, C=C, C-N, and O-C=O groups, providing adsorption sites for water. However, in SG 1.00, excess rGO coated the SiO 2 and decreased the number of adsorption sites, which might be the reason why the adsorption isotherm differed from that of SG 0.30. The water adsorption amounts were possibly related to the concentration of oxygen and nitrogen functional groups, such as C-O-C, C-N, and O-C=O, because these groups have the highest affinity toward water molecules [33]. SG 0.30 had a higher concentration of oxygen and nitrogen functional groups than SG 1.00; therefore, the amount of adsorbed water on SG 0.30 was higher than that on SG 1.00.
To investigate the effect of defects on the adsorption properties of the materials, Raman spectroscopy was used. The D band and G band were observed in all samples at shifts of 1350 and 1590 cm −1 , respectively, representing disordered and ordered graphitic carbons. The ratio of the D band to G band intensity is shown in Figure 7. The ratio of I d /I g is a sensitive characterization of the disorder of a tested sample, with higher values indicating a higher disorder and more obvious defects and voids. The number of defects of rGO and the rGO/SiO 2 nanocomposites increased as GO was reduced. Compared with that of rGO and SG1.00, the highest I d /I g was obtained with SG 0.30. It is possible that the abundant defects of SG 0.30 provided more adsorption sites for cationic NR adsorption. amounts were possibly related to the concentration of oxygen and nitrogen functional groups, such as C-O-C, C-N, and O-C=O, because these groups have the highest affinity toward water molecules [33]. SG 0.30 had a higher concentration of oxygen and nitrogen functional groups than SG 1.00; therefore, the amount of adsorbed water on SG 0.30 was higher than that on SG 1.00. To investigate the effect of defects on the adsorption properties of the materials, Raman spectroscopy was used. The D band and G band were observed in all samples at shifts of 1350 and 1590 cm −1 , respectively, representing disordered and ordered graphitic carbons. The ratio of the D band to G band intensity is shown in Figure 7. The ratio of Id/Ig is a sensitive characterization of the disorder of a tested sample, with higher values indicating a higher disorder and more obvious defects and voids. The number of defects of rGO and the rGO/SiO2 nanocomposites increased as GO was reduced. Compared with that of rGO and SG1.00, the highest Id/Ig was obtained with SG 0.30. It is possible that the abundant defects of SG 0.30 provided more adsorption sites for cationic NR adsorption.   To investigate the effect of defects on the adsorption properties of the materials, Raman spectroscopy was used. The D band and G band were observed in all samples at shifts of 1350 and 1590 cm −1 , respectively, representing disordered and ordered graphitic carbons. The ratio of the D band to G band intensity is shown in Figure 7. The ratio of Id/Ig is a sensitive characterization of the disorder of a tested sample, with higher values indicating a higher disorder and more obvious defects and voids. The number of defects of rGO and the rGO/SiO2 nanocomposites increased as GO was reduced. Compared with that of rGO and SG1.00, the highest Id/Ig was obtained with SG 0.30. It is possible that the abundant defects of SG 0.30 provided more adsorption sites for cationic NR adsorption.

Adsorption Equilibrium and Kinetics of SiO 2 and the rGO/SiO 2 Composites
Herein, to investigate the removal ability of SiO 2 and rGO/SiO 2 , we used NR adsorption (pH 5) at 298 K. The parameters of the pseudo-first-order kinetic and pseudo-second-order kinetic models are listed in Tables 1 and 2, respectively. The correlation coefficients of the pseudo-second-order model are all higher than 0.920, and the pseudo-second-order model agrees well with the experimental values. Therefore, the adsorption of NR on SiO 2 and rGO/SiO 2 composites was more consistent with the pseudo-second-order kinetic model.  Figure 8 shows the effect of the ratio of GO and SiO 2 on the adsorption rate of NR (C 0 = 10 mg/L). For the samples at different ratios, the adsorption equilibrium with NR could be achieved within nearly 120 min, and no significant change was observed from 120 to 360 min. As the amount of rGO increased, the adsorption efficiency and adsorption capacity first increased and then decreased. Clearly, compared with the other samples, SG 0.30 was the first sample to reach adsorption equilibrium (30 min), and displayed the highest experimental adsorption amount (12.853 mg g −1 ). The reason the adsorption rate of SG 0.30 was faster than that of SG 0.50 and SG 1.00 might be that rGO and silica formed a better synergistic interaction to form more "core-shell" structures in SG 0.30. As the amount of rGO increased, excess rGO began to agglomerate, hindering the adsorption of NR and leading to a decrease in the adsorption rate. Moreover, the reason the adsorption capacity of SG 0.05 was lower than that of SiO 2 might be the absence of a "core-shell" structure and the poor affinity of rGO toward organic contaminants [34]. and silica formed a better synergistic interaction to form more "core-shell" structures in SG 0.30. As the amount of rGO increased, excess rGO began to agglomerate, hindering the adsorption of NR and leading to a decrease in the adsorption rate. Moreover, the reason the adsorption capacity of SG 0.05 was lower than that of SiO2 might be the absence of a "core-shell" structure and the poor affinity of rGO toward organic contaminants [34]. To further test the adsorption property of SG 0.30, the concentration of NR was increased to 25 and 50 mg/L (pH = 5). For comparison, SiO2 was used as a reference. Figure 9a shows the removal efficiency and saturated amount of adsorbed NR at equilibrium for SiO2 and SG 0.30 versus the initial NR concentration at room temperature. At the same time, it was obvious that the removal efficiency of SG 0.30 was greater than that of SiO2. Figure 9b displays the saturated amount of adsorbed NR at equilibrium versus different initial NR concentrations (10,25, and 50 mg/L) at room temperature. When compared, the experimental adsorption capacities of SG 0.30 were 1.876, 3.791, and 1.353 times higher than those of SiO2. In Table 3, the maximum adsorption amounts of the composites toward the adsorption of NR dye from the aqueous solution are compared with those of other adsorbents in the literature at room temperature. As clearly seen, the rGO/SiO2 nanocomposites in this study exhibited a superior adsorption capacity and have potential for use in the treatment of wastewater containing cationic NR dye. To further test the adsorption property of SG 0.30, the concentration of NR was increased to 25 and 50 mg/L (pH = 5). For comparison, SiO 2 was used as a reference. Figure 9a shows the removal efficiency and saturated amount of adsorbed NR at equilibrium for SiO 2 and SG 0.30 versus the initial NR concentration at room temperature. At the same time, it was obvious that the removal efficiency of SG 0.30 was greater than that of SiO 2 . Figure 9b displays the saturated amount of adsorbed NR at equilibrium versus different initial NR concentrations (10, 25, and 50 mg/L) at room temperature. When compared, the experimental adsorption capacities of SG 0.30 were 1.876, 3.791, and 1.353 times higher than those of SiO 2 . In Table 3, the maximum adsorption amounts of the composites toward the adsorption of NR dye from the aqueous solution are compared with those of other adsorbents in the literature at room temperature. As clearly seen, the rGO/SiO 2 nanocomposites in this study exhibited a superior adsorption capacity and have potential for use in the treatment of wastewater containing cationic NR dye. the amount of rGO increased, excess rGO began to agglomerate, hindering the adsorption of NR and leading to a decrease in the adsorption rate. Moreover, the reason the adsorption capacity of SG 0.05 was lower than that of SiO2 might be the absence of a "core-shell" structure and the poor affinity of rGO toward organic contaminants [34]. To further test the adsorption property of SG 0.30, the concentration of NR was increased to 25 and 50 mg/L (pH = 5). For comparison, SiO2 was used as a reference. Figure 9a shows the removal efficiency and saturated amount of adsorbed NR at equilibrium for SiO2 and SG 0.30 versus the initial NR concentration at room temperature. At the same time, it was obvious that the removal efficiency of SG 0.30 was greater than that of SiO2. Figure 9b displays the saturated amount of adsorbed NR at equilibrium versus different initial NR concentrations (10, 25, and 50 mg/L) at room temperature. When compared, the experimental adsorption capacities of SG 0.30 were 1.876, 3.791, and 1.353 times higher than those of SiO2. In Table 3, the maximum adsorption amounts of the composites toward the adsorption of NR dye from the aqueous solution are compared with those of other adsorbents in the literature at room temperature. As clearly seen, the rGO/SiO2 nanocomposites in this study exhibited a superior adsorption capacity and have potential for use in the treatment of wastewater containing cationic NR dye.

Conclusions
In summary, a porous rGO/SiO 2 nanocomposite was synthesized using a simple surface grafting method, which shows potential for mass production in industry. The obtained rGO/SiO 2 nanocomposite had a porous structure containing micro-and meso-pores. Water vapor adsorption/desorption indicated that the oxygen and nitrogen functional groups, such as the C-C, C-O-C, C=C, C-N, and O-C=O groups on the surfaces of the nanocomposites, provided adsorption sites for water because of their high affinity toward water molecules. On the other hand, the synthesis of rGO/SiO 2 nanocomposites exhibited a novel process for the fast liquid-phase removal of cationic neutral red (NR) dye at room temperature. Furthermore, rGO/SiO 2 exhibited a higher adsorption capacity than SiO 2 and other similar materials used for the removal of NR dye. Therefore, the rGO/SiO 2 nanocomposites show potential for use in the treatment of wastewater containing dyes; however, the removal time, reaction conditions, and cost for contaminant treatment need to be optimized in future works.