Self-Assembly Reduced Graphene Oxide Nanosheet Hydrogel

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The Self-Assembly Reduced Graphene Oxide Nanosheet Hydrogel Fabrication by Anchorage of Chitosan/Silver and Its Potential Efficient Application toward Dyes Degradation for Wastewater Treatments Ti-Feng Jiao, Heng Zhao, Jingxin Zhou, Qingrui Zhang, Xiaona Luo, Jie Hu, Qiuming Peng, and Xuehai Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00695 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 22, 2015

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ACS Sustainable Chemistry & Engineering

The Self-Assembly Reduced Graphene Oxide Nanosheet Hydrogel Fabrication by Anchorage of Chitosan/Silver and Its Potential Efficient Application toward Dyes Degradation for Wastewater Treatments Tifeng Jiao,*a,b,c Heng Zhao,b Jingxin Zhou,b Qingrui Zhang,*b Xiaona Luo,b Jie Hu,b Qiuming Penga and Xuehai Yanc

a

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,

Qinhuangdao 066004, P. R. China. b

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

Yanshan University, Qinhuangdao 066004, P. R. China. c

National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, P. R. China.

Correspondence and requests for materials should be addressed to T. Jiao ([email protected]), and Q. Zhang ([email protected]).

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ABSTRACT In this study, the hydrogel materials of reduced graphene oxide (RGO)/chitosan/silver nanoparticle composites were designed and prepared via self-assembly process and simultaneous reduction of chitosan molecules with GO. These as-prepared hydrogels were characterized by different techniques. The morphology of the internal network structure of the nanocomposite hydrogels was investigated. The catalytic capacity results demonstrate that the prepared GO-based composite hydrogels can efficiently remove two tested dye molecules from wastewater in well accordance with the pseudo-second-order model. The dye photocatalytic capacity of the obtained hydrogels is mainly attributed to the silver nanoparticle on RGO sheets, whereas the chitosan molecule was incorporated to facilitate the gelation process of the GO sheets. Interestingly, the as-prepared catalytic composite material serve as a good photocatalyst for present used two dyes even for dye mixture, suggesting the potentially real situation applications of the GO composite materials for wastewater treatment as well as the removal of harmful dyes.

KEYWORDS: Graphene oxide, Composite hydrogel, Chitosan, Dyes degradation, Wastewater treatment

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INTRODUCTION Nowadays harmful dyes and poisonous substance have been attracting a lot of interest. It seemed that efficient removal of toxic compounds and organic dyes from various wastewater had been a big problem for most researchers1-4. For instance, O. Akhavan and coworkers have successfully reported the synthesis of some new ZnFe2O4/RGO composite materials via hydrothermal reaction and investigated the separation and photothermal application through magnetic behaviors5,6. In addition, 3D porous gels from graphene composites show new properties, such as large surface areas, high compressibility, ultralow density, and strong mechanical strength7-14. For example, A. Banerjee’s groups successfully achieved the synthesis of various hydrogels that can be used as dye-adsorbing agents in wastewater removal15,16. In addition, this group has also reported metal nanoparticle/GO composite hydrogels and novel morphological transformation of graphene based nanohybrids17-19. Generally, GO-based composite hydrogels were obtained by mixing organic macromolecules or small amphiphiles with an aqueous GO dispersion20-22. Although some systems about GO-based composite hydrogels for catalyst materials have been investigated,23-26 the design and reusing of the photocatalyst materials after wastewater purification have been a challenging problem. In addition, among many reported templates, mesoporous networks in GO composite hydrogel structure are particularly special and appropriate for the nanoparticle formation in comparison with the traditional non-aqueous prepared routes27-33. For example, Gao and coworkers investigated some GO nanocomposite hydrogel with 3D network nanostructure in hybrid materials33. On the other hand, chitosan (CS), the well-known compound of chitin N-deacetylation, shows many eco-friendly properties, such as biodegradation, good

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biocompatibility, and antifungal activity. These characteristics make it a promising candidate in the fields of catalysis, materials, food, drugs, etc.34-37. Since GO crosslinks with chitosan via the carboxyl, hydroxyl, epoxy, and –NH2 groups of chitosan, the adsorption capacity of the formed composite is expected to be high. To date, some reports have been published on GO-chitosan nanocomposites used for drug release and antimicrobial activity38-42. In this study, we report the preparation of GO-based composite hydrogels using the self-assembly of CS and GO, and an in situ reduction approach. The composite hydrogels consist of both hydrogen bonding and electrostatic interactions between the CS molecules and GO sheets. CS molecules were incorporated to facilitate the gelation process of GO sheets, and the dye catalytic capacity of the hydrogel was mainly attributed to the silver nanoparticle on GO sheets. For the two dyes tested in this study, namely, methylene blue (MB), and Rhodamine B (RhB), the as-prepared composite hydrogels exhibit good removal rates in well accordance with the pseudo-second-order model. More importantly, the hydrogels prepared in this study have potential large-scale applications in organic dye removal and wastewater treatment.

EXPERIMENTAL SECTION Materials CS (> 90% degree of deacetylation) was obtained from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). RhB and MB were purchased from Tianjin KaiTong Chemical Reagent co., Ltd (Tianjin, China). Other materials, such as silver nitrate, sulfuric acid (98%), vitamin C, potassium permanganate (KMnO4), graphite powder (99.85% purity), hydrogen peroxide (30%, w/w), potassium nitrate (KNO3), and hydrochloric acid, were obtained from Aladdin Chemicals (Tianjin, China) and used without

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further purification. The water used in all the experiments was obtained using a Milli-Q ultrapure water purification system.

Fabrication of the RGO/CS/Ag Hydrogel GO was obtained from graphite powder according to a modified Hummers method43. Brown GO sheets were obtained after centrifugation and freeze drying at −50°C for 2−3 days. GO-based composite hydrogels, including RGO/CS hydrogel and RGO/CS/Ag hydrogel, were prepared using CS. In a typical experiment, 4.0 mL of an aqueous dispersion of GO (5 mg/mL) was mixed with 0.4 mL of CS (30.0 mg/mL, prepared in 2.5% acetic acid solution) in a glass vial. In order to obtain a homogeneous gelation state, the hydrogel was then sonicated for 2−3 min. For the preparation of the RGO/CS composite hydrogel, 1.0 mL of vitamin C solution (60 mg/mL) was first added into 4.0 mL of a GO aqueous dispersion (5 mg/mL), which was stirred for several minutes. Next, 0.4 mL of CS solution (30.0 mg/mL, prepared in 2.5% acetic acid solution) was added to the above solution and the obtained ternary mixture was sonicated for about 10 min and heated subsequently at 90°C for 10 min. After heating, the GO component in the gel transformed to RGO with slight shrinking of the gel state. In addition, for the preparation of RGO/CS/Ag hydrogel, 1.0 mL of aqueous AgNO3 solution (25 mg/mL) was mixed with 4.0 ml of aqueous GO dispersion (5 mg/mL). Then the obtained solution was stirred continuously. And 1.0 mL of vitamin C solution (100 mg/mL) was added to the above mixed solution, followed by the addition of 0.4 ml of CS solution (30.0 mg/mL, prepared in 2.5% acetic acid solution). This obtained mixed solution was sonicated for about 10 min and heated subsequently at 90°C for 10 min. In this process, silver ions were reduced by the ascorbic acid to form Ag nanoparticles in gel nanostructures.

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Batch Photocatalytic Tests for Dyes Removal Catalytic capacity experiments were designed and measured at room temperature. In experimental process, about 1 mL of as-obtained RGO/CS/Ag hydrogel (without lyophilization) was added to 350 mL of each dye solution (MB, 10 mg/L; RhB, 4 mg/L). For the preparation of mixed dye solution, the above MB and RhB solutions were mixed with volume ratio of 1:1. These solutions were slowly stirred in dark at a constant rate at room temperature. Then, the obtained solution was irradiated by a high pressure mercury lamp (365 nm, 100 W) with 15 cm distance from light source to solution surface. Supernatant liquid was withdrawn at different time intervals for subsequent characterization using an UV-Vis spectrometer (752, Sunny Hengping, Shanghai, China). Different absorbance wavelengths (662 nm for MB, and 554 nm for RhB) were utilized to determine the concentration of the residual dyes in the supernatant liquid. The calculation formula of dye degradation rate (K) was the following equation (1):

K=

( A0 − AT ) ⋅100% A0

(1)

Where K is defined as the degradation rate; AO is defined as the absorbance of the original solution; AT is defined as the absorbance of the solution at any time.

Characterization The GO sheets and xerogels used in the present study were prepared using a lyophilizer at −50 °C with

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a FD-1C-50 lyophilizer instrument from Beijing Boyikang Experimental Instrument Co., Ltd. (Beijing, China) for more than 2 days. The nanostructures of all the lyophilized samples were studied by a field-emission scanning electron microscope (SEM) (S-4800II, Hitachi, Japan) with 5−15 kV accelerating voltage as well as a transmission electron microscope (TEM) (HT7700, Hitachi High-Technologies Corporation, Japan). The chemical composition in present materials was measured by energy-dispersive X-ray spectroscopy (EDXS). EDXS investigation was obtained at 200 kV accelerating voltage via an Oxford Link-ISIS X-ray EDXS microanalysis system attached to TEM. A Shimadzu UV-2550 system (Shimadzu Corporation, Japan) was utilized to measure the UV-vis spectra. Atomic force microscope (AFM) data were performed via a Nanoscope model Multimode 8 Scanning Probe

Microscope

(Veeco

Instrument,

USA)

with

silicon

cantilever

probes.

Thermogravimetry-differential scanning calorimetry (TG-DSC) characterizations were carried out by using a NETZSCH STA 409 PC Luxxsi multaneous thermal analyzer (Netzsch Instruments Manufacturing Co., Ltd., Germany) in air. X-ray diffraction (XRD) was conducted on an X-ray diffractometer equipped with a Cu Kα X-ray radiation source and a Bragg diffraction setup (SMART LAB, Rigaku, Japan). FTIR spectra were obtained by a Fourier infrared spectroscopy (Thermo Nicolet Corporation) via the KBr tablet method. Raman spectroscopy was measured via a Horiba Jobin Yvon Xplora PLUS confocal Raman microscope equipped with a motorized sample stage. The wavelength of the excitation laser was 532 nm and the laser power was maintained below 1 mW without noticeable sample heating. X-ray photoelectron spectroscope (XPS) was carried on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. Both survey scans and individual high-resolution scans for Ag(3d), N(1s), O(1s) and C(1s) peaks were measured. The photocatalytic process was measured by a HPLC system (Agilent Technologies 1200 Series) equipped with a diode array detector and an auto-sampler. Chromatographic separations were performed at room temperature

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using an Agilent Eclipse XD8-C18 column (4.5 µm, 4.6*150 mm). The used mobile phases contain 80% methanol and 20% water with a flow rate 0.9 mL min-1.

RESULTS AND DISCUSSION Preparation and Characterization of RGO Based Hydrogel. The photographs of the GO aqueous solution, GO/CS gel, RGO/CS gel, and RGO/CS/Ag composite gels are demonstrated in Fig. 1h. It indicates that all gels show good gelation stability. Both the SEM and TEM pictures of the obtained composite hydrogels in Fig. 1 indicate that porous 3D net-like nanostructures were formed by the self-assembly of GO sheets and CS molecules. It should be noted that after in situ reduction of GO by vitamin C under heating (90 °C), the GO composite gel can transform to a RGO-based hydrogel. It is well known that vitamin C usually acts as an eco-friendly moderate reducing agent. At the same time, the formed composite gels can provide enough space among its 3D nanostructure for the adsorption and degradation of organic dyes. In addition, the in situ formed silver nanoparticles were homogeneously anchored on RGO surface to form a ternary nanocomposite material, as shown in Figure 1c and 1g. The EDXS were used to characterize the chemical components in RGO/CS/Ag composite hydrogel, as shown in Figure 1d. The chemical signals of C, Ag, and Cu elements were obtained. The appearance of Cu peaks is mainly due to the TEM grid. The result of EDXS indicated that silver nanostructures were obtained in composite due to the reduction of vitamin C. It is interesting to note that the diameters of Ag nanoparticles are mainly in the range of 60-80 nm, which can be mainly due to in situ reduction of silver salts in composite hydrogel nanostructures. In addition, AFM characterizations were also utilized to measure the RGO/CS and RGO/CS/Ag hydrogel composites (shown in Figure 2). The obtained pictures clearly demonstrated the

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formation of 3D nanostructured sheets. The main driving forces to fabricate present organized 3D network-like nanostructure are various weak interactions between building blocks, such as hydrogen bonding and π-π stacking44,45. The picture in Figure 2B indicated the as-prepared silver nanoparticles are homogeneously anchored and fixed on RGO sheet surface in composite gels. In order to characterize structural changes in composite materials, FT-IR spectra of GO sheet and all composite hydrogels were measured, as shown in Figure 3A. As for the spectrum of GO sheet, the characteristic peak appeared at 3424 cm-1 can be assigned to the -OH vibration stretching. In addition, other obvious bands demonstrated carboxyl group at 1724 cm-1, epoxy group at 1226 cm-1, and alkoxy group at 1050 cm-1. These characteristic functional groups usually exist on surface or at edge of GO nanosheets46-48. Moreover, for the spectrum of GO/CS hydrogel, obvious peak appeared at 1645 cm-1, which could be mainly attributed to the -NH stretching in CS molecules. However, for the spectra of RGO/CS and RGO/CS/Ag composite materials, the typical peaks, such as -NH stretching of CS, carboxyl C=O and C-O, and alkoxy C-O groups can be clearly observed, while the peak assigned to epoxy C-O at 1226 cm-1 disappeared. In addition, obvious amide peaks could be clearly observed at 1640 and 1385 cm-1. Thus, the present obtained IR data demonstrated the transformation process of GO to RGO and successful synthesis Ag nanoparticle-containing RGO-based composite materials. In addition, XRD was used to characterize the obtained organized nanostructures in the composite materials, as shown in Figure 3B. For the GO sheet and lyophilized GO/CS hydrogel samples, the peaks that appear at 11.1° and 9.8° respectively indicate increased layer spacing between the GO layers in the organized composite gels. In the XRD pattern of the dried RGO composite hydrogel, a broad peak centered at 2θ value of 22.6° is clearly seen and the diffraction peak at 2θ value of 11.1° in the GO gel disappears. These changes suggest that the transformation from GO to RGO is accompanied by

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the removal of various oxygen-containing functional segments on the GO surface49. As for obtained RGO/CS/Ag hydrogel material, many diffraction peaks appeared with 2θ values of 17.6°, 38.2°, 44.3°, 64.4°, and 77.5°. The first diffraction peak with 2θ value of 17.6° indicate the transformation from GO to RGO in composite material. Other diffraction peaks with 2θ values of 38.2°, 44.3°, 64.4°, and 77.5° are in well accordance with Ag standard card (JCPDS card no. 04-0783, space group Fm-3m(225)), which could be clearly assigned to the characteristic (111), (200), (220), and (311) Miller indices of Ag, respectively. Figure 4 demonstrates the thermograms of the obtained composite gel materials. It can be concluded from the TG data that all the composite hydrogels demonstrate better thermal stability compared with GO; this could mainly be due to the self-assembly and cross-linked nanostructures in the gels. The data indicates two obvious mass loss peaks at around 200 and 500°C for present obtained composite materials. The results mainly originate from the pyrolysis of the oxygen-containing segments in the GO sheets and CS molecules, respectively. At temperatures higher than 560°C, no further change is observed in the mass of the prepared RGO gel material. And the quality value of RGO/CS/Ag hydrogel material is clearly higher than that of RGO/CS composite hydrogel. In addition, it should be noted that GO composites have been previously reported to demonstrate various weight retention ratios at high temperature range, mainly due to the structural change in the self-assembly process of the 3D porous carbon composites50-52. In the present study, the as-prepared composite materials clearly exhibit enhanced thermal stability in comparison with pristine GO. It is well known that Raman spectroscopy is a useful technique to investigate various carbon composite materials53. Now Raman spectra for the composites in the present study are shown in Figure 5. The spectrum of GO sheets shows three obvious bands at 1601 cm-1, 1351 cm-1, and 2692 cm-1, which can

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be assigned to the G band, D band, and 2D band, respectively54,55. It has been reported that the G and 2D bands can shift to lower and higher wavenumbers from their original positions at 1585 and 2679 cm-1, respectively, when single-layer graphene sheets are transformed to the multi-layer (about 2–6 layers) state56,57. On the other hand, the 2D/G ratios for graphene sheets with different layers (1, 2, 3, >4) are normally >1.6, 0.8, 0.30, and 0.07, respectively58,59. Recently Akhavan and coworkers have reported single and bilayer GO sheets with 2D/G ratios in the range of 1.53–1.68 and 0.82–0.89, respectively60. For the present study, the 2D/G ratios in all composite materials were calculated to have values ranging from 0.11 to 0.14, shown in Fig. 5C, indicating that our as-prepared graphene samples possessed multilayer structures. Furthermore, it is also well known that the D/G peak intensity ratio can serve as a characteristic of the sp2 area size of graphene sheets containing sp3 and sp2 bonds. In the present case, the results indicate that the D/G ratio value changes from 0.96 to 1.02−1.14 after the self-assembly in composite gels, as seen in Fig. 5B. In addition, the surface compositions of as-prepared nanocomposite materials have been investigated by XPS technique. The survey XPS spectra of all materials (seen in Figure 6A and 6B) demonstrated main characteristic peaks of C(1s), N(1s), and O(1s). In addition, the obtained O/C ratios are clearly different (GO sheet, 38.42%; GO/CS gel, 42.09%; RGO/CS gel, 40.63%; and RGO/CS/Ag gel, 41.26%). This indicated the change process from GO to RGO and the simultaneous decrement of oxygen element. Then, the deconvolution of XPS peaks for the RGO/CS/Ag composite material, such as C(1s), N(1s), O(1s), and Ag(3d), were investigated in details. Firstly, as for deconvolution of C(1s) shown in Figure 6C, the peak centered at 284.8 eV was mainly assigned to the contribution of C-C, C=C and C-H bonds. At the same time, other deconvoluted peaks appeared at values of 286.6, 287.4, 288.2 and 289.2 eV could originate from various oxygen-containing bonds, such as C-OH, C-O-C, C=O, and O=C-OH bonds, respectively61. In addition, the N(1s) peak shown in Figure 6D indicated the

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appearance of amine group at about 399.4 eV and N+ segments at about 401.4 eV, demonstrating the self-assembly of CS molecules in composite gels via both covalent bonding and weak interaction force to the GO sheets62. Moreover, the O(1s) peak (seen in Figure 6E) can be deconvoluted into two Gaussian component peaks after a Shirley backline correction. A component peak at 532.0 eV can be due to the oxygen of surface OH¯ linked in the composite nanostructure. This seemed to be helpful to generate OH⋅ free radicals in photocatalysis processes63,64. Another deconvoluted O(1s) peak at 533.0 eV was mainly assigned to the oxygen in water molecules adsorbed in the composite nanostructure. This result indicated the obtained porous RGO/CS/Ag nanocomposite material was suitable to apply in next photocatalytic fields. Next, the Ag(3d) spectrum of the RGO/CS/Ag gel was shown in Figure 6F. The Ag(3d5/2) and Ag(3d3/2) peaks appeared at 368.1 and 374.1 eV, respectively. It is interesting to note that the slitting of the 3d doublet of Ag is 6.0 eV, suggesting the successful synthesis of metallic silvers in present RGO/CS/Ag gels64. Moreover, the deconvolution of the Ag(3d) peak was also investigated. The present Ag(3d5/2) peak could be deconvoluted to three Gaussian component peaks after a Shirley backline correction. Three deconvoluted peaks of Ag(3d5/2) appeared at 368.4, 368.1 and 367.6 eV for Ag, Ag2O and AgO, respectively. The deconvolution results indicated that 68% of the silvers were in Ag0 chemical state, with 6% and 26% of silvers in Ag+ and Ag2+ chemical states, respectively.

Photocatalytic Performances toward Dye Degradation. The dye catalytic capacity of the as-prepared Ag nanoparticle-containing composite hydrogels was investigated and evaluated by placing the obtained composite gel materials in aqueous MB and RhB solutions, respectively. It should be noted that the adsorption process of freeze-dried samples seemed typical and easy to investigate. In present work the in situ degradation capacity of hydrogels were

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chosen, which could demonstrate the real degradation process of GO-based gels for different dyes in wastewater. In addition, graphene oxide-based composites have the probability to act as a visible light photocatalyst and degradation of dyes in adsorption process. So the present catalytic experiments were measured and repeated in dark and light condition, respectively. Figure 7 demonstrates the calculated dye removal rate versus time plots for both used dyes with the utilization of Ag nanoparticles-containing composite gel. The value of dye degradation rates can get nearly 100% for MB within 70 mins time, indicating well efficiency of the as-obtained RGO/CS/Ag composite material as good photocatalysts. On the same time, in control experiment without UV light irradiation, the removal results of the same composite gels was significantly reduced. In addition, the degradation kinetic experiments of the as-obtained RGO/CS/Ag composite materials on both used dyes were measured, and the experimental data were demonstrated in Figure 8. The nanocomposite hydrogels demonstrated continuous and homogeneous adsorption process, with equilibrium times of around 40 min for MB and 80 min for RhB, respectively. The obtained kinetic results can mainly originate from the prepared special 3D porous nanostructures by electrostatic force and hydrogen bonding, as well as highly dispersed Ag nanoparticles anchored on RGO sheets surface as active photocatalytic locations. It should be noted that classical kinetic models were used to explain the degradation process as follows:

The pseudo-first-order model can be described by equation (2): log(qe - qt ) = logqe -

k t 2.303

(2)

The pseudo-second-order model can be described by equation (3): t 1 t = + 2 q t k qe qe

(3)

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where qe and qt are defined as the degraded dye amount (mg/g) at equilibrium and time t, respectively, and k1 and k2 represent the kinetic rate constants65. From the kinetic data in Table 1, it can be concluded that the present degradation process in all cases is in accordance with the pseudo-second-order model with good correlation coefficient (R2 > 0.992). Moreover, it should be noted that in order to simulate the real situation with mixture of dyes in waste water, the photocatalytic capacity of present obtained RGO/CS/Ag composite materials on degradations of present mixed solution of MB and RhB at 298 K have also been measured, seen in Figure S1-S4 and Table S1 in Supporting Information. The degradation kinetics curves of present RGO/CS/Ag composite gels on mixed dye solutions of MB and RhB indicated present obtained materials can serve as a good photocatalyst even for mixture dyes in accordance with pseudo-second-order model with good correlation coefficient. Thus, the reasonable charge transfer process and mechanism that happened in present Ag nanoparticle-containing composite materials are demonstrated in Figure 9. During the catalytic process, used dye molecules have been transferred and adsorbed onto graphene sheet with organized packing or orientation via π–π stacking between dye skeleton and aromatic regions of carbon net66. While UV light was used to irradiate the prepared RGO/CS/Ag composite gel, the obtained photo-excited electrons can quickly shift to surface of graphene components and push some adsorbed O2 molecules to generate O2and O22- radicals67. Under this condition, present composite material could produce more electrons and holes, and next obtain more superoxide anions and/or peroxide species68. As the predictable result, used dye molecules are decomposed to intermediate products and finally H2O, CO2 and other mineralization. In addition, it should be noted that the photocatalytic study has been investigated by using UV-visible spectroscopy and high performance liquid chromatography (HPLC) (seen in Figure S5 and S6 in the

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Supporting Information). It is clearly demonstrated in the Figure S5, the decrease in intensity of the 664 nm peak for MB has been obtained with extended light irradiation time in the presence of composite material. In HPLC result, the change process of the MB degradation using present Ag nanoparticle-containing composite has been measured at various time intervals. The HPLC retention time at 3.1 min and 2.0-2.1 min are corresponding to the MB and catalytic intermediate, respectively. At initial time (t = 0) strong peak has been obtained at retention time of 3.1 min for MB. With the degradation progress, this peak has become continuously decreased with time progress and nearly disappeared at time t = 100 min. And the catalytic intermediate at 2.0-2.1 min (retention time) appeared at t = 5-10 min and gradually decreased with the progress of time. The present HPLC data clearly gives direct evidence of photodegradation dye for present as-obtained composite materials. Furthermore, the present obtained results can be compared with the report by A. Banerjee’s group, in which Au nanoparticle-containing RGO composite hydrogel material can catalyze hydrogenation reactions17. On the other hand, separation and regeneration of catalytic nanoparticles seemed very important and becoming big problems in industrial applications. In our recent other research work, the designed preparation and the dye removal capacity of GO/Fe3O4 nanocomposites and core–shell MnO2 nanocomposites have been investigated in details, in which the obtained inorganic composite materials can be controlled recycled or reused for several times, supporting long time utilization in wastewater purification69,70. However, as for present RGO/CS/Ag composite material, due to the mass loss of CS in regeneration process by washing by organic solvents, as well as the possible oxidation of Ag nanoparticles in drying process, the regeneration capacity and reusability seemed not so well. It seemed still a challenge to design and improve the regeneration capacity of organic-inorganic composite materials.

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CONCLUSIONS In summary, we reported the facile design and synthesis of RGO/CS/Ag composite gel material, and evaluated the dye degradation capacity for two different dyes. The CS molecule was chosen for its functional amine segments in the molecular skeleton that can form porous gel nanostructures via interactions such as hydrogen bonding. Morphological characterizations of the obtained gel materials demonstrate the formation of porous 3D nanostructures by a self-assembly process. At the same time, the in situ formed silver nanoparticles appeared uniformly anchored on GO sheets surface to obtain a ternary nanocomposite system. The data of photocatalytic capacity experiments suggest that the prepared 3D GO-based hydrogels can efficiently remove dyes and exhibit well photocatalytic performances for present used RhB and MB single solution or mixed solution in accordance with the pseudo-second-order model. We anticipate that the GO-based composite hydrogels prepared in the present study could open up new avenues for the design and application of composite hydrogels materials.

ASSOCIATED CONTENT Supporting Information Photocatalytic properties, Photos, degradation kinetics curves, kinetic parameters for mixed dye solution degradations, UV-vis spectra, and HPLC analyses. The Supporting Information is available free of charge on the ACS Publications website at DOI:XXXX.

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AUTHOR INFORMATION Corresponding Authors E-mail address: [email protected] (Jiao TF); and [email protected] (Zhang QR) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by NSFC (grant Nos. 21473153, 21207112 and 51578476), NSF of Hebei Province (grant No. B2013203108), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (grant No. YQ2013026), the Support Program for the Top Young Talents of Hebei Province, the Scientific and Technological Research and Development Program of Qinhuangdao City (grant No. 201502A006), the Open Foundation of National Key Laboratory of Biochemical Engineering (Institute of Process Engineering, Chinese Academy of Sciences), and Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science (grant No. RAE2014CE03B).

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Table 1. Kinetic parameters of RGO/CS/Ag nanocomposite for MB and RhB degradations and removal at 298 K (experimental data from Figure 8).

Pseudo-first-order model MB qe

R2

(mg/g)

Pseudo-second-order model

K1

qe

(min-1)

(mg/g)

R2

K2 (g/mg·min)

UV light

161.063

0.76817

0.31035

168.919

0.99981

0.00360

Light avoidance

153.381

0.8409

0.14185

166.945

0.99941

0.00137

Pseudo-first-order model RhB qe

R2

(mg/g)

Pseudo-second-order model

K1

qe

(min-1)

(mg/g)

R2

K2 (g/mg·min)

UV light

61.638

0.96001

0.05945

68.259

0.99888

0.00129

Light avoidance

28.967

0.96624

0.16049

29.976

0.99994

0.01108

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Figure captions Figure 1. SEM and TEM images for the lyophilized GO/CS hydrogel (a, e), RGO/CS hydrogel (b, f), and RGO/CS/Ag hydrogel (c, g), respectively. Image d is EDXS taken on the RGO/CS/Ag hydrogel shown in g. Image h is photographs: GO aqueous solution, GO/CS, RGO/CS, and RGO/CS/Ag composite hydrogels (from left to right). Figure 2. AFM images of lyophilized RGO/CS gels (A) and RGO/CS/Ag gels (B). Figure 3. IR spectra (A) and XRD patterns (B) of lyophilized samples: a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel. Figure 4. TG curves of lyophilized samples: a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel. Figure 5. Raman spectroscopy of lyophilized samples (A): a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel. (B) and (C) present D/G and 2D/G ratios of the Raman spectra shown in (A), respectively. Figure 6. Survey XPS spectra of lyophilized samples: (A) GO sheet; (B) a, GO/CS gel; b, RGO/CS gel; c, RGO/CS/Ag gel. Deconvolution of XPS peaks of the RGO/CS/Ag nanocomposite: C, C(1s); D, N(1s); E, O(1s); F, Ag(3d). Figure 7. Photocatalytic properties of RGO/CS/Ag gel on MB (a) and RhB (b) solution, respectively. The inserted photos are dye solutions acquired for the supernatant liquids collected at different time intervals during photocatalytic experiment. Figure 8. Degradation kinetics curves of as-prepared RGO/CS/Ag nanocomposites on MB (a, b) and RhB (c, d) at 298 K. Figure 9. Proposed scheme of photocatalytic degradation of RGO/CS/Ag gel on dye solution.

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Figure 1. SEM and TEM images for the lyophilized GO/CS hydrogel (a, e), RGO/CS hydrogel (b, f), and RGO/CS/Ag hydrogel (c, g), respectively. Image d is EDXS taken on the RGO/CS/Ag hydrogel shown in g. Image h is photographs: GO aqueous solution, GO/CS, RGO/CS, and RGO/CS/Ag composite hydrogels (from left to right).

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Figure 2. AFM images of lyophilized RGO/CS gels (A) and RGO/CS/Ag gels (B).

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Figure 3. IR spectra (A) and XRD patterns (B) of lyophilized samples: a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel.

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Figure 4. TG curves of lyophilized samples: a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel.

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Figure 5. Raman spectroscopy of lyophilized samples (A): a, GO sheet; b, GO/CS gel; c, RGO/CS gel; d, RGO/CS/Ag gel. (B) and (C) present D/G and 2D/G ratios of the Raman spectra shown in (A), respectively.

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Figure 6. Survey XPS spectra of lyophilized samples: (A) GO sheet; (B) a, GO/CS gel; b, RGO/CS gel; c, RGO/CS/Ag gel. Deconvolution of XPS peaks of the RGO/CS/Ag nanocomposite: C, C(1s); D, N(1s); E, O(1s); F, Ag(3d).

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Figure 7. Photocatalytic properties of RGO/CS/Ag gel on MB (a) and RhB (b) solution, respectively. The inserted photos are dye solutions acquired for the supernatant liquids collected at different time intervals during photocatalytic experiment.

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Figure 8. Degradation kinetics curves of as-prepared RGO/CS/Ag nanocomposites on MB (a, b) and RhB (c, d) at 298 K.

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Figure 9. Proposed scheme of photocatalytic degradation of RGO/CS/Ag gel on dye solution.

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For Table of Contents Use Only.

The Self-Assembly Reduced Graphene Oxide Nanosheet Hydrogel Fabrication by Anchorage of Chitosan/Silver and Its Potential Efficient Application toward Dyes Degradation for Wastewater Treatments

Tifeng Jiao, Heng Zhao, Jingxin Zhou, Qingrui Zhang, Xiaona Luo, Jie Hu, Qiuming Peng and Xuehai Yan

Synopsis A reduced graphene oxide/chitosan/silver nanoparticle composite hydrogel has been demonstrated as photocatalyst for organic dye removal and wastewater treatment.

35


Self-Assembly Reduced Graphene Oxide Nanosheet Hydrogel

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