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Facile synthesis of soluble functional graphene by reduction of graphene oxide via acetylacetone and its adsorption of heavy metal ions
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Nanotechnology Nanotechnology 25 (2014) 395602 (9pp)
doi:10.1088/0957-4484/25/39/395602
Facile synthesis of soluble functional graphene by reduction of graphene oxide via acetylacetone and its adsorption of heavy metal ions Minghan Xu, Jing Chai, Nantao Hu, Da Huang, Yuxi Wang, Xiaolu Huang, Hao Wei, Zhi Yang and Yafei Zhang Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China E-mail: [email protected] and [email protected] Received 3 May 2014, revised 24 July 2014 Accepted for publication 4 August 2014 Published 11 September 2014 Abstract
The synthesis of graphene (GR) from graphene oxide (GO) typically involves harmful chemical reducing agents that are undesirable for most practical applications. Here we report a green and facile synthesis method for the synthesis of GR that is soluble in water and organic solvents and that includes the additional benefit of adsorption of heavy metal ions. Acetylacetone, as both a reducing agent and a stabilizer, was used to prepare soluble GR from GO. Transmission electron microscopy and atomic force microscopy provide clear evidence for the formation of few-layer GR. The results from Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy show that reduction of GO to GR has occurred. Raman spectroscopy and X-ray photoelectron spectroscopy also indicate the removal of oxygen-containing functional groups from GO, resulting in the formation of GR. The results of dispersion experiments show that GR can be highly dispersed in water and N,N-Dimethylformamide. The reaction mechanism for acetylacetone reduction of exfoliated GO was also proposed. This method is a facile and environmentally friendly approach to the synthesis of GR and opens up new possibilities for preparing GR and GRbased nanomaterials for large-scale applications. Of even greater interest is that inductively coupled plasma atomic emission spectroscopy suggests that synthesized GR may be applied in the absorption of Cd2+ and Co2+ due to the strong coordination capacity of acetylacetone on the surfaces and edges of GR and the large surface area of GR in aqueous solutions. The maximum adsorptions are 49.28 mg g−1 for Cd2+, which is 4.5 times higher than that of carbon nanotubes, and 27.78 mg g−1 for Co2+, which is 3.6 times higher than that of titania beans. Keywords: graphene, acetylacetone, graphene oxide, adsorption 1. Introduction
[4], fuel cells [5], analytical sensors [6], and so on. In recent research, the aforementioned analytical sensors have mostly been used in the detection of metal ions [7] and other molecules such as H2O2 [8] by using the electrochemical or fluorescent properties of the functional groups in GR [9, 10]. GR can act as an electrode or as macromolecules, directly detecting trace amounts of metal ions or target molecules. However, there is little research regarding adsorption of metal
Ever since graphene (GR) was discovered in 2004, it has been one of the most sought-after materials in the scientific community because of its unique chemical and physical properties, such as large surface area (theoretical surface area as high as 2630 m2 g−1) and high electrical conductivity (calculated to be about 6400 S cm−1) [1, 2]. It can be applied in dye-sensitized solar cells [3], catalysis of Knoevenagel condensation 0957-4484/14/395602+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
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Figure 1. Schematic diagram of reduction of GO using Hacac and its ion adsorption.
and Co2+. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is used to measure the adsorption amounts of Cd2+ and Co2+. A schematic diagram of reduction of GO using Hacac and its ion adsorption is shown in figure 1.
ions by means of the coordination capacity of the reducing agent. Common synthetic methods for GR have been documented in numerous articles; these methods include exfoliation [11], chemical vapor deposition (CVD) [12], arc discharge [13], and reduction [14]. Common reduction methods include, among others, electrochemical reduction [15], photocatalytic reduction [16], thermal reduction [17], and chemical reduction [18]. Moreover, the normal chemical reduction method is used to prepare GR, that is, by the reduction of graphene oxide (GO) using reducing agents such as hydrazine, dimethylhydrazine, and sodium borohydride [19]. However, these reducing agents are highly toxic, so trace amounts of these poisonous agents can have a detrimental effect, especially in biomedical applications. Therefore, environmentally friendly chemical reduction methods urgently need to be developed. In addition, a common mechanism for chemical reduction is that the reducing agents lose electrons, whereas GO sheets act as electron-accepting monomers [20]. Chemical reduction is a large-scale and economical way to prepare GR. There are various functional groups or reducing agents on the surfaces or edges of the obtained GR sheets. The molecules that bind to GR with unique properties, such as fluorescence or coordination, can be used to absorb or detect metal ions. In this paper, acetylacetone (Hacac), whose pKa is about 8 ∼ 9 [21, 22], is used to reduce GO to GR in an alkaline environment for the first time. This method has many advantages. For example, Hacac is cheap and safe, no hazardous waste is generated, the process does not require a tedious procedure, and the colloidal route is currently considered the most attractive option for many other prospective applications of GR. Hacac is used as a coordination agent to form coordination complexes that can be used as precursors to prepare nanoparticles [23] and detect heavy metal ions [24]. In addition, Hacac possesses strong coordination capacity with respect to most inorganic metal ions—especially heavy metal ions—so Hacac functionalized on the surfaces and edges of GR may be used to absorb the heavy metal ions Cd2+
2. Experiment details 2.1. Materials
High-purity natural graphite (99.99%, 500 mesh) was obtained from Qingdao Tianyuan Graphite Co., Ltd (Shandong, China). All chemicals were of analytical-reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China), without further purification. Deionized water with a resistivity of 18.1 MΩ cm was used for all experiments. 2.2. Preparation of GR with Hacac
GO was prepared using a modified Hummers method [25, 26], which was similar to our previous work [27, 28]. The typical procedure was as follows: 2 g of graphite and 50 mL of concentrated sulfuric acid were added into a 250 mL flask. The mixture was stirred vigorously for 30 min. Next, 5 g of NaNO3 were added, followed by cooling of the mixture to 0 °C using an ice water bath. After continuous stirring for 2 h, 7.3 g of KMnO4 were added in small portions over the course of 1 h. Subsequently, the system temperature was increased to 35 °C. The reaction was allowed to take place for 2 h and was then quenched by adding 0.2 L of ice water and 7.0 mL of H2O2 (30%). The resultant GO was filtered and washed with plenty of aqueous HCl (3%) until no precipitation of BaSO4 occurred in the presence of the aqueous BaCl2 solution. Further washing with water was carried out until the chloride test with AgNO3 was negative. The resultant products were dried at 40 °C for 24 h in a vacuum oven. GR was reduced from GO by using Hacac as the reducing agent. Typically, 0.10 g GO was sonicated in 250 mL 2
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Figure 2. (a) Normalized UV–vis spectra of GO aqueous dispersions before (0 h) and after being reduced via Hacac and ammonia for 2 h, 4 h, 8 h, 12 h, 24 h, and 48 h. (b) Enlarged UV–vis spectra of (a).
deionized water. 2 mL Hacac were added into this solution, and then the pH of the solution was adjusted to 10 using ammonia aqueous solution. Subsequently, the solution was continuously stirred at 80 °C for 24 h. Next as-prepared GR was washed with water, acetone, and ethanol several times and finally dried in air at 50 °C overnight.
Fourier transform infrared spectrometer (FT-IR, VERTEX 70, Bruker Optics, Germany), with a detection range of 4000 to 400 cm−1. Ultraviolet-visible spectra (UV–vis, Lambda 500, PerkinElmer, USA) characterizations were carried out in the region from 250 to 800 nm. Thermogravimetric analysis (TGA, Pyris 1 TGA, PerkinElmer, USA) characterizations were used to analyze the thermal stability of graphite, GO, and GR in a nitrogen atmosphere under heat treatment from 30 to 900 °C. The heating rates of graphite, GO, and GR are 10, 1, and 10 °C min−1, respectively. ICP-AES (iCAP 6000 Radial, Thermo Fisher Scientific Inc., USA) was used to detect the concentrations of Cd2+ and Co2+. X-ray photoelectron spectroscopy (XPS, PH1500C, Japan) was used to detect surface elements of GO and GR.
2.3. Adsorption of heavy metal ions 2.3.1. Single metal ion equilibrium adsorption. 50 mg GR
were individually added to a series of 100 mL Cd2+ and Co2+ aqueous solutions with different concentrations from 0.2 to 40 mg L−1 for Cd2+ and 0.2 to 50 mg L−1 for Co2+. The pH value of the solutions was adjusted to 6.0. The suspension was allowed to remain magnetically stirred for 4 h. Then the solution was filtered with a 0.22 μm filter membrane. The concentrations of Cd2+ and Co2+ in the filtrate were detected by ICP-AES.
3. Results and discussion Reduction of GO under different control conditions was carried out. GO aqueous solutions were added separately with Hacac, ammonia, and Hacac and ammonia at room temperature, 40 °C, 60 °C, and 80 °C for 24 h. It was found that the solution obviously turned black after the addition of Hacac and ammonia into the GO aqueous solutions at 80 °C for 24 h. At other temperatures and conditions, the color of the solution either did not change or changed slightly. Moreover, the reason for choosing 10.0 as the pH value of the solution was that Hacac can be transferred easily from keto to enol under alkaline conditions. We took UV–vis measurements to monitor the degree of reduction in the control experiments. As shown in figure 2, as the reduction progressed, the intensity of the UV–vis absorption peak was centered at 300 nm (this was caused by the GO), decayed gradually with the increase in reaction time, and disappeared after 24 h. Meanwhile, the absorption peak shown around 231 nm was redshifted with the increase in reduction time [25]. The dispersion of 0.7 mg mL−1 GR in different solvents is shown in figure 3. The solubility of Hacac-reduced GR in water is the best among all solvents. In nonpolar solvents such as acetone, the solubility is poorer, which is probably attributable to the interactions of nonpolar solvent molecules and
2.4. Effect of pH on adsorption
The concentrations of Cd2+ and Co2+ were maintained at 40 mg L−1 and 30 mg L−1, respectively. 50 mg GR were individually added to this solution by adjusting the pH values from 3.0 to 9.0. Then the solution was magnetically stirred for 4 h and filtered with a 0.22 μm filter membrane. The concentrations of Cd2+ and Co2+ in the filtrate were detected by ICP-AES. The aforementioned adsorption experiments were carried out at room temperature. 2.5. Characterization
The morphology of GR was analyzed by field emission scanning electron microscope (SEM, Carl Zeiss Ultra 55, Germany) at 5 kV, transmission electron microscopy (TEM, JEM-2100, Japan), and atomic force microscopy (AFM, ESweep, SEIKO, Japan). GR was dispersed in water and then deposited on mica substrate for AFM measurement. For TEM observation, GR was dispersed in water and then deposited on 300 mesh carbon-coated Cu grids. Raman spectra were obtained by dispersive Raman microscope (Senterra R200-L, Bruker Optics, Germany) with an excitation wavelength at 532 nm. The functional groups in GR were analyzed by 3
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Figure 3. Digital photo of GR dispersed in different solvents (from left to right): N,N-Dimethylformamide (DMF), acetone, water, ethanol,
0
1
2
[um]
3
4
5
and isopropanol (IPA) after sonication for a period of 30 min.
0
1
2
[um] 3
4
5 2.60 [nm]
0
[nm]
2129.436 1.62
Figure 4. (a) SEM image of GR film. (b) AFM image of GR on mica substrate (top) and height profile of the AFM image (bottom). (c) Lowmagnification TEM image of GR sheets. (d) High-resolution TEM image of GR sheets. The inset in (d) is a typical SAED pattern.
The thickness as measured by AFM is shown in figure 4(b), which illustrates that the thickness of the GR is about 0.7 nm as measured from the height profile. Although the interlayer spacing of graphite is 0.34 nm in theory, practical AFM measurements of a single GR layer on substrate always lead to high values due to the weak sample/substrate interactions and the presence of ambient species (nitrogen, oxygen, argon, or water) between the substrate and the GR [29]. Thus, based on GR thicknesses reported in other works [25, 30], as-prepared GR is considered to be monolayer.
the functional groups (such as Hacac) of GR. The homogeneous colloidal suspensions of GR are easily obtained in water and can be stable for several months. The morphologies of GR were investigated by SEM, AFM, and TEM. The SEM image of solid GR in figure 4(a) shows that two or more layers are overlapped and wrinkled to display a silklike appearance and that a large number of grooves are formed by extrusion in order to reduce the surface energy of the GR. The size of the surface area of the GR is several tens of micrometers in length. 4
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Figure 5. (a) Raman spectra of GR and GO; (b) FT-IR spectra of graphite, GO, and GR; (c) UV–vis spectra of GO, GR, and Hacac dispersed
in water; and (d) TGA curves of graphite, GO, and GR.
TEM images and the selected area electron diffraction (SAED) pattern shown in figures 4(c) and (d) illustrate that the structural feature of as-prepared GR is that of a sheet. The low-magnification TEM image shown in figure 4(c) displays the wrinkled morphology of the GR, which may be attributable to the interaction between GR sheets. The size of an asprepared GR sheet is several micrometers. The high-resolution TEM image in figure 4(d) shows that GR sheets tend to be stacked together. The SAED pattern of GR shows that the diffraction pattern is relatively diffused, which may be due to the fact that GR nanosheets are not perpendicular to the electron beam [31] or that chemically reduced GR has a few defects and the reducing agent molecules are functionalized on its surfaces and edges. The structural changes of GR and GO were characterized by Raman spectra as shown in figure 5(a). Two typical feature peaks of GR are exhibited in the Raman spectra. The peak at 1344 cm−1 is the D mode of GR, which arises from the breathing mode of the A1g symmetry. The peak at 1596 cm−1 is G mode, arising from first-order scattering of the E2g photon of sp2 C atoms. Change in the relative intensity between the D and G bands (ID/IG) indicates the change in the electronic conjugation state of the GO during the reduction process [32]. The value of ID/IG increases from 0.89 to 0.99 in the reduction process; this increase is ascribed to a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO. Another possible interpretation is that new graphitic domains are smaller but more numerous [32]. Besides, the 2D peak of GR located at 2693 cm−1 is broad, indicating that the solid state of GR is assembled [33, 34].
Functional groups of GO and GR can be corroborated and deduced from FT-IR spectra in figure 5(b). The peaks of GO are situated at 3447 and 1633 cm−1, corresponding to hydroxyl groups on the surface of GO and aromatic C=C bonds, respectively. The peak at 997 cm−1 is ascribed to the = C-H out-of-plane wagging vibration. The IR spectrum of GR at 3500 ∼ 3240 cm−1 and 1580 cm−1, corresponding to associated hydroxyls and the stretching vibration of C=O, which is blueshifted due to C=C conjugated to C=O. It is verified that Hacac acts as functional groups of as-prepared GR. The intensities of the bands associated with oxygen functional groups greatly decrease relative to those of unreduced GO. UV–vis spectra are shown in figure 5(c). The maximum absorption peak of GO, which is situated at 300 nm, is attributable to π-π* electron transition of C=O bonds. The characteristic absorption peak of Hacac located at 274 nm is attributable to conjugated C=C and π-π*, or n-π* electron transition. The absorption peak of as-prepared GR is blueshifted, attributable to Hacac functionalized on the surfaces and edges of GR. The thermal stability of pristine graphite, GO, and GR was analyzed by TGA in a nitrogen atmosphere. As shown in figure 5(d), the weight of the graphite starts to decrease at 617 °C due to the decomposition of the carbon skeleton. It can be seen that the curve of GO has three drops. The weight losses at 137 and 207 °C are ascribed to the decomposition of labile oxygen functional groups of as-prepared GO, yielding CO, CO2, and H2O [35]. The weight loss curve of GO starting at 538 °C is lower than that of GR, which demonstrates that 5
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Figure 6. C1s XPS spectra of (a) GO and (b) GR.
Figure 7. The formation mechanism of GR by reduction of GO via Hacac.
there are more functional groups in GO than in GR. Moreover, the first drop in the GR curve at 178 °C is attributed to the hydroxyl groups on the basal plane of the GR. It exhibits only a little loss at 254 °C, which is much lower than for GO, indicating a significant decrease in oxygenated functional groups. The later steep drop at 594 °C is caused by the pyrolysis of the remaining oxygen-containing groups as well as the burning of the carbon skeleton. The decomposition temperature of GR (594 °C) is higher than that of GO (538 °C), indicating the removal of the thermally labile oxygen functional groups by chemical reduction, which leads to the enhanced thermal stability of GR. Figure 6 shows the C1s XPS spectra of GO and GR. Before reduction, four different peaks shown in figure 6(a) centered at 284.8, 286.9, 287.8, and 289.1 eV, corresponding to C=C/C-C in aromatic rings, C-O (epoxy and alkoxy), C=O, and COOH groups, respectively, are detected. After the reduction with Hacac and ammonia for 24 h, as shown in figure 6(b), the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of C-O (epoxy and alkoxy), decreased dramatically, revealing that oxygen-
containing functional groups are removed after reduction [25]. In addition, a new peak at 285.5 eV, corresponding to the C-N group, appears due to the collaborative reduction of Hacac and ammonia [36]. The formation mechanism of GR by reduction of GO via Hacac is shown in figure 7. Epoxy groups of GR are changed into the double bonds, leaving Hacac molecules to increase the stability of GR [32].
3.1. Adsorption of heavy metal ions
Single metal ion equilibrium adsorption studies were conducted to investigate the maximum metal adsorption capacities of carbon nanotubes. The adsorption isotherm of Cd2+ and Co2+ by obtained GR is shown in figure 8(a). It is observed that the amount of adsorbed Cd2+ and Co2+ increases significantly in the range of low equilibrium concentration; it then increases gradually and reaches 51 mg g−1 for Cd2+ and 26 mg g−1 for Co2+ at an equilibrium concentration of 15 mg L−1.
6
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2+
Figure 8. (a) The adsorption isotherm of GR for Cd
and Co2+ at pH 6.0 and (b) the effect of pH on the adsorption of Co2+ and Cd2+ by GR. 2+
Table 1. Langmuir parameters of single adsorption isotherm models
Table 2. Comparison of individual Cd
for Cd
capacities of different adsorbents.
2+
2+
and Co
in GR.
Adsorbent
Cd2+ (mg g−1)
Co2+ (mg g−1)
Graphene
49.28
27.78
Carbon nanotubes Biosorbent
10.86 24.3
— —
Granular AC2
3.37
—
Powdered AC
3.37
—
Titania beans Fe3O4/MnO2 Nanocomposite PHEMA3
8.94 53.2
7.62 —
—
0.74
P(MMA-HEMA)4
—
28.84
Single ion adsorption Types of ions Cd2+ Co2+
qm (mg g−1)
b (L mg−1)
r2
49.28 27.78
7.8 3.1
0.99 902 0.99 578
A linear Langmuir adsorption isotherm model is used to calculate the maximum adsorption amount of Cd2+ and Co2+. The Langmuir absorption equation is as follows:
Ce Ce 1 = + qe bqm qm where Ce stands for the equilibrium concentration of Cd2+ or Co2+ (mg/L), qe is the adsorption amount (mg/g), qm stands for the maximum adsorption capacity corresponding to complete monolayer coverage (mg/g), and b represents the Langmuir constant indirectly related to the energy of adsorption (L/mg). The Langmuir parameters of GR adsorption of Cd2+ and Co2+ are calculated and summarized in table 1. The correlation coefficients, r2, are high and close to 1, suggesting that the experimental data can be represented by the Langmuir adsorption and that the adsorbed metal ions form monolayer coverage on the surfaces of the GR. According to the Langmuir equation, the maximum adsorption qm of Cd2+ is 49.28 mg g−1, the Langmuir constant b equals 7.8, and the correlation coefficient r2 is 0.99 902. Compared with the maximum adsorption of Cd2+ by carbon nanotubes of 10.86 mg g−1, the adsorption capacity of GR is 4.5 times higher [37]. Similarly, the maximum adsorption amount of Co2+ is 27.78 mg g−1, the Langmuir constant b equals 3.1, and the correlation coefficient r2 is 0.99 578. Compared with the maximum adsorption of Co2+ by titania beans of 7.62 mg g−1, the adsorption capacity of GR is 3.6 times higher [38]. The higher adsorption capacity of Cd2+ and Co2+ for as-prepared GR may be attributed to the strong coordination capacity of Hacac and the large surface area of GR in aqueous solutions. The effect of pH on the adsorption of Cd2+ and Co2+ in GR is shown in figure 8(b). When the pH value is lower
and Co2+ adsorption
Conditions
Refs.
pH 6.0; RT1 pH 5.0; RT pH 4.7; 293 K pH 5.0; 303 K pH 5.0; 303 K pH 5.9; RT PH 6.3; RT
our data 37 39
pH 6; 303 K pH 6; 303 K
39 39 38 40 41 41
1
RT stands for room temperature. AC stands for activated carbon. 3 PHEMA stands for poly 2-hydroxyethyl methacrylate. 4 P(MMA-HEMA) stands for copolymer 2-hydroxyethyl methacrylate with monomer methyl methacrylate. 2
than 5.0, the adsorption capacity increases with the increase in pH, which leads mainly to the conclusion that the enol form of Hacac occurs prior to the forming of the coordination complex with Cd2+ and Co2+ in GR. However, the adsorption amount remains almost unchanged as pH values increase from 5.0 to 6.0. Therefore, the pH value 6.0 is chosen as the optimum condition. As the pH values increase from 6.0 to 9.0, the adsorption capacity increases gradually; this may be due to the combined role of adsorption and precipitation. The adsorption capacities of Cd2+ and Co2+ by GR were compared with those of other adsorbents (table 2, with the data collected under different conditions). The Cd2+ adsorption capacity of Fe3O4/MnO2 nanocomposite [40] and the Co2+ adsorption capacity of copolymer 2-hydroxyethyl methacrylate [41] with monomer methyl methacrylate are higher than that of our GR. The metal removal capacities of the other adsorbents such as carbon nanotubes, biosorbent [39], granular 7
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activated carbon (AC), powdered AC, titania beans, and poly 2-hydroxyethyl methacrylate are lower than those of our GR. The comparison suggests that GR has great potential for use in heavy metal ion adsorbents in wastewater treatment.
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4. Conclusions In summary, a green and facile approach to preparing soluble GR has been developed via a new reducing agent, Hacac, which acts as both reducing agent and stabilizer during the formation of GR. Moreover, Hacac is an excellent coordination agent that can coordinate with most metal ions. The maximum adsorption of obtained GR is 49.28 mg g−1 for Cd2+, which is 4.5 times higher than that of carbon nanotubes, and 27.78 mg g−1 for Co2+, which is 3.6 times higher than that of titania beans. The greater capacity of GR to adsorb Cd2+ and Co2+ may be attributed to the strong coordination capacity of Hacac on the surfaces and edges of GR and the large surface area of GR in aqueous solutions. Therefore, we expect that our findings will further the preparation of high-quality GR and GR-based nanocomposites and significantly facilitate the application of GR in water pollution treatment and environmental protection.
Acknowledgments The authors gratefully acknowledge the financial support of the National High-Tech R & D Program of China (863 program, 2011AA050504); the National Natural Science Foundation of China (51102164 and 61376003); the Program for New Century Excellent Talents in University (NCET-120356); Shanghai Science and Technology Grant (12nm0503800), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University.
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Facile synthesis of soluble functional graphene by reduction of graphene oxide via acetylacetone and its adsorption of heavy metal ions
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Nanotechnology Nanotechnology 25 (2014) 395602 (9pp)
doi:10.1088/0957-4484/25/39/395602
Facile synthesis of soluble functional graphene by reduction of graphene oxide via acetylacetone and its adsorption of heavy metal ions Minghan Xu, Jing Chai, Nantao Hu, Da Huang, Yuxi Wang, Xiaolu Huang, Hao Wei, Zhi Yang and Yafei Zhang Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China E-mail: [email protected] and [email protected] Received 3 May 2014, revised 24 July 2014 Accepted for publication 4 August 2014 Published 11 September 2014 Abstract
The synthesis of graphene (GR) from graphene oxide (GO) typically involves harmful chemical reducing agents that are undesirable for most practical applications. Here we report a green and facile synthesis method for the synthesis of GR that is soluble in water and organic solvents and that includes the additional benefit of adsorption of heavy metal ions. Acetylacetone, as both a reducing agent and a stabilizer, was used to prepare soluble GR from GO. Transmission electron microscopy and atomic force microscopy provide clear evidence for the formation of few-layer GR. The results from Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy show that reduction of GO to GR has occurred. Raman spectroscopy and X-ray photoelectron spectroscopy also indicate the removal of oxygen-containing functional groups from GO, resulting in the formation of GR. The results of dispersion experiments show that GR can be highly dispersed in water and N,N-Dimethylformamide. The reaction mechanism for acetylacetone reduction of exfoliated GO was also proposed. This method is a facile and environmentally friendly approach to the synthesis of GR and opens up new possibilities for preparing GR and GRbased nanomaterials for large-scale applications. Of even greater interest is that inductively coupled plasma atomic emission spectroscopy suggests that synthesized GR may be applied in the absorption of Cd2+ and Co2+ due to the strong coordination capacity of acetylacetone on the surfaces and edges of GR and the large surface area of GR in aqueous solutions. The maximum adsorptions are 49.28 mg g−1 for Cd2+, which is 4.5 times higher than that of carbon nanotubes, and 27.78 mg g−1 for Co2+, which is 3.6 times higher than that of titania beans. Keywords: graphene, acetylacetone, graphene oxide, adsorption 1. Introduction
[4], fuel cells [5], analytical sensors [6], and so on. In recent research, the aforementioned analytical sensors have mostly been used in the detection of metal ions [7] and other molecules such as H2O2 [8] by using the electrochemical or fluorescent properties of the functional groups in GR [9, 10]. GR can act as an electrode or as macromolecules, directly detecting trace amounts of metal ions or target molecules. However, there is little research regarding adsorption of metal
Ever since graphene (GR) was discovered in 2004, it has been one of the most sought-after materials in the scientific community because of its unique chemical and physical properties, such as large surface area (theoretical surface area as high as 2630 m2 g−1) and high electrical conductivity (calculated to be about 6400 S cm−1) [1, 2]. It can be applied in dye-sensitized solar cells [3], catalysis of Knoevenagel condensation 0957-4484/14/395602+09$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
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Figure 1. Schematic diagram of reduction of GO using Hacac and its ion adsorption.
and Co2+. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is used to measure the adsorption amounts of Cd2+ and Co2+. A schematic diagram of reduction of GO using Hacac and its ion adsorption is shown in figure 1.
ions by means of the coordination capacity of the reducing agent. Common synthetic methods for GR have been documented in numerous articles; these methods include exfoliation [11], chemical vapor deposition (CVD) [12], arc discharge [13], and reduction [14]. Common reduction methods include, among others, electrochemical reduction [15], photocatalytic reduction [16], thermal reduction [17], and chemical reduction [18]. Moreover, the normal chemical reduction method is used to prepare GR, that is, by the reduction of graphene oxide (GO) using reducing agents such as hydrazine, dimethylhydrazine, and sodium borohydride [19]. However, these reducing agents are highly toxic, so trace amounts of these poisonous agents can have a detrimental effect, especially in biomedical applications. Therefore, environmentally friendly chemical reduction methods urgently need to be developed. In addition, a common mechanism for chemical reduction is that the reducing agents lose electrons, whereas GO sheets act as electron-accepting monomers [20]. Chemical reduction is a large-scale and economical way to prepare GR. There are various functional groups or reducing agents on the surfaces or edges of the obtained GR sheets. The molecules that bind to GR with unique properties, such as fluorescence or coordination, can be used to absorb or detect metal ions. In this paper, acetylacetone (Hacac), whose pKa is about 8 ∼ 9 [21, 22], is used to reduce GO to GR in an alkaline environment for the first time. This method has many advantages. For example, Hacac is cheap and safe, no hazardous waste is generated, the process does not require a tedious procedure, and the colloidal route is currently considered the most attractive option for many other prospective applications of GR. Hacac is used as a coordination agent to form coordination complexes that can be used as precursors to prepare nanoparticles [23] and detect heavy metal ions [24]. In addition, Hacac possesses strong coordination capacity with respect to most inorganic metal ions—especially heavy metal ions—so Hacac functionalized on the surfaces and edges of GR may be used to absorb the heavy metal ions Cd2+
2. Experiment details 2.1. Materials
High-purity natural graphite (99.99%, 500 mesh) was obtained from Qingdao Tianyuan Graphite Co., Ltd (Shandong, China). All chemicals were of analytical-reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China), without further purification. Deionized water with a resistivity of 18.1 MΩ cm was used for all experiments. 2.2. Preparation of GR with Hacac
GO was prepared using a modified Hummers method [25, 26], which was similar to our previous work [27, 28]. The typical procedure was as follows: 2 g of graphite and 50 mL of concentrated sulfuric acid were added into a 250 mL flask. The mixture was stirred vigorously for 30 min. Next, 5 g of NaNO3 were added, followed by cooling of the mixture to 0 °C using an ice water bath. After continuous stirring for 2 h, 7.3 g of KMnO4 were added in small portions over the course of 1 h. Subsequently, the system temperature was increased to 35 °C. The reaction was allowed to take place for 2 h and was then quenched by adding 0.2 L of ice water and 7.0 mL of H2O2 (30%). The resultant GO was filtered and washed with plenty of aqueous HCl (3%) until no precipitation of BaSO4 occurred in the presence of the aqueous BaCl2 solution. Further washing with water was carried out until the chloride test with AgNO3 was negative. The resultant products were dried at 40 °C for 24 h in a vacuum oven. GR was reduced from GO by using Hacac as the reducing agent. Typically, 0.10 g GO was sonicated in 250 mL 2
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Figure 2. (a) Normalized UV–vis spectra of GO aqueous dispersions before (0 h) and after being reduced via Hacac and ammonia for 2 h, 4 h, 8 h, 12 h, 24 h, and 48 h. (b) Enlarged UV–vis spectra of (a).
deionized water. 2 mL Hacac were added into this solution, and then the pH of the solution was adjusted to 10 using ammonia aqueous solution. Subsequently, the solution was continuously stirred at 80 °C for 24 h. Next as-prepared GR was washed with water, acetone, and ethanol several times and finally dried in air at 50 °C overnight.
Fourier transform infrared spectrometer (FT-IR, VERTEX 70, Bruker Optics, Germany), with a detection range of 4000 to 400 cm−1. Ultraviolet-visible spectra (UV–vis, Lambda 500, PerkinElmer, USA) characterizations were carried out in the region from 250 to 800 nm. Thermogravimetric analysis (TGA, Pyris 1 TGA, PerkinElmer, USA) characterizations were used to analyze the thermal stability of graphite, GO, and GR in a nitrogen atmosphere under heat treatment from 30 to 900 °C. The heating rates of graphite, GO, and GR are 10, 1, and 10 °C min−1, respectively. ICP-AES (iCAP 6000 Radial, Thermo Fisher Scientific Inc., USA) was used to detect the concentrations of Cd2+ and Co2+. X-ray photoelectron spectroscopy (XPS, PH1500C, Japan) was used to detect surface elements of GO and GR.
2.3. Adsorption of heavy metal ions 2.3.1. Single metal ion equilibrium adsorption. 50 mg GR
were individually added to a series of 100 mL Cd2+ and Co2+ aqueous solutions with different concentrations from 0.2 to 40 mg L−1 for Cd2+ and 0.2 to 50 mg L−1 for Co2+. The pH value of the solutions was adjusted to 6.0. The suspension was allowed to remain magnetically stirred for 4 h. Then the solution was filtered with a 0.22 μm filter membrane. The concentrations of Cd2+ and Co2+ in the filtrate were detected by ICP-AES.
3. Results and discussion Reduction of GO under different control conditions was carried out. GO aqueous solutions were added separately with Hacac, ammonia, and Hacac and ammonia at room temperature, 40 °C, 60 °C, and 80 °C for 24 h. It was found that the solution obviously turned black after the addition of Hacac and ammonia into the GO aqueous solutions at 80 °C for 24 h. At other temperatures and conditions, the color of the solution either did not change or changed slightly. Moreover, the reason for choosing 10.0 as the pH value of the solution was that Hacac can be transferred easily from keto to enol under alkaline conditions. We took UV–vis measurements to monitor the degree of reduction in the control experiments. As shown in figure 2, as the reduction progressed, the intensity of the UV–vis absorption peak was centered at 300 nm (this was caused by the GO), decayed gradually with the increase in reaction time, and disappeared after 24 h. Meanwhile, the absorption peak shown around 231 nm was redshifted with the increase in reduction time [25]. The dispersion of 0.7 mg mL−1 GR in different solvents is shown in figure 3. The solubility of Hacac-reduced GR in water is the best among all solvents. In nonpolar solvents such as acetone, the solubility is poorer, which is probably attributable to the interactions of nonpolar solvent molecules and
2.4. Effect of pH on adsorption
The concentrations of Cd2+ and Co2+ were maintained at 40 mg L−1 and 30 mg L−1, respectively. 50 mg GR were individually added to this solution by adjusting the pH values from 3.0 to 9.0. Then the solution was magnetically stirred for 4 h and filtered with a 0.22 μm filter membrane. The concentrations of Cd2+ and Co2+ in the filtrate were detected by ICP-AES. The aforementioned adsorption experiments were carried out at room temperature. 2.5. Characterization
The morphology of GR was analyzed by field emission scanning electron microscope (SEM, Carl Zeiss Ultra 55, Germany) at 5 kV, transmission electron microscopy (TEM, JEM-2100, Japan), and atomic force microscopy (AFM, ESweep, SEIKO, Japan). GR was dispersed in water and then deposited on mica substrate for AFM measurement. For TEM observation, GR was dispersed in water and then deposited on 300 mesh carbon-coated Cu grids. Raman spectra were obtained by dispersive Raman microscope (Senterra R200-L, Bruker Optics, Germany) with an excitation wavelength at 532 nm. The functional groups in GR were analyzed by 3
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Figure 3. Digital photo of GR dispersed in different solvents (from left to right): N,N-Dimethylformamide (DMF), acetone, water, ethanol,
0
1
2
[um]
3
4
5
and isopropanol (IPA) after sonication for a period of 30 min.
0
1
2
[um] 3
4
5 2.60 [nm]
0
[nm]
2129.436 1.62
Figure 4. (a) SEM image of GR film. (b) AFM image of GR on mica substrate (top) and height profile of the AFM image (bottom). (c) Lowmagnification TEM image of GR sheets. (d) High-resolution TEM image of GR sheets. The inset in (d) is a typical SAED pattern.
The thickness as measured by AFM is shown in figure 4(b), which illustrates that the thickness of the GR is about 0.7 nm as measured from the height profile. Although the interlayer spacing of graphite is 0.34 nm in theory, practical AFM measurements of a single GR layer on substrate always lead to high values due to the weak sample/substrate interactions and the presence of ambient species (nitrogen, oxygen, argon, or water) between the substrate and the GR [29]. Thus, based on GR thicknesses reported in other works [25, 30], as-prepared GR is considered to be monolayer.
the functional groups (such as Hacac) of GR. The homogeneous colloidal suspensions of GR are easily obtained in water and can be stable for several months. The morphologies of GR were investigated by SEM, AFM, and TEM. The SEM image of solid GR in figure 4(a) shows that two or more layers are overlapped and wrinkled to display a silklike appearance and that a large number of grooves are formed by extrusion in order to reduce the surface energy of the GR. The size of the surface area of the GR is several tens of micrometers in length. 4
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Figure 5. (a) Raman spectra of GR and GO; (b) FT-IR spectra of graphite, GO, and GR; (c) UV–vis spectra of GO, GR, and Hacac dispersed
in water; and (d) TGA curves of graphite, GO, and GR.
TEM images and the selected area electron diffraction (SAED) pattern shown in figures 4(c) and (d) illustrate that the structural feature of as-prepared GR is that of a sheet. The low-magnification TEM image shown in figure 4(c) displays the wrinkled morphology of the GR, which may be attributable to the interaction between GR sheets. The size of an asprepared GR sheet is several micrometers. The high-resolution TEM image in figure 4(d) shows that GR sheets tend to be stacked together. The SAED pattern of GR shows that the diffraction pattern is relatively diffused, which may be due to the fact that GR nanosheets are not perpendicular to the electron beam [31] or that chemically reduced GR has a few defects and the reducing agent molecules are functionalized on its surfaces and edges. The structural changes of GR and GO were characterized by Raman spectra as shown in figure 5(a). Two typical feature peaks of GR are exhibited in the Raman spectra. The peak at 1344 cm−1 is the D mode of GR, which arises from the breathing mode of the A1g symmetry. The peak at 1596 cm−1 is G mode, arising from first-order scattering of the E2g photon of sp2 C atoms. Change in the relative intensity between the D and G bands (ID/IG) indicates the change in the electronic conjugation state of the GO during the reduction process [32]. The value of ID/IG increases from 0.89 to 0.99 in the reduction process; this increase is ascribed to a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO. Another possible interpretation is that new graphitic domains are smaller but more numerous [32]. Besides, the 2D peak of GR located at 2693 cm−1 is broad, indicating that the solid state of GR is assembled [33, 34].
Functional groups of GO and GR can be corroborated and deduced from FT-IR spectra in figure 5(b). The peaks of GO are situated at 3447 and 1633 cm−1, corresponding to hydroxyl groups on the surface of GO and aromatic C=C bonds, respectively. The peak at 997 cm−1 is ascribed to the = C-H out-of-plane wagging vibration. The IR spectrum of GR at 3500 ∼ 3240 cm−1 and 1580 cm−1, corresponding to associated hydroxyls and the stretching vibration of C=O, which is blueshifted due to C=C conjugated to C=O. It is verified that Hacac acts as functional groups of as-prepared GR. The intensities of the bands associated with oxygen functional groups greatly decrease relative to those of unreduced GO. UV–vis spectra are shown in figure 5(c). The maximum absorption peak of GO, which is situated at 300 nm, is attributable to π-π* electron transition of C=O bonds. The characteristic absorption peak of Hacac located at 274 nm is attributable to conjugated C=C and π-π*, or n-π* electron transition. The absorption peak of as-prepared GR is blueshifted, attributable to Hacac functionalized on the surfaces and edges of GR. The thermal stability of pristine graphite, GO, and GR was analyzed by TGA in a nitrogen atmosphere. As shown in figure 5(d), the weight of the graphite starts to decrease at 617 °C due to the decomposition of the carbon skeleton. It can be seen that the curve of GO has three drops. The weight losses at 137 and 207 °C are ascribed to the decomposition of labile oxygen functional groups of as-prepared GO, yielding CO, CO2, and H2O [35]. The weight loss curve of GO starting at 538 °C is lower than that of GR, which demonstrates that 5
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Figure 6. C1s XPS spectra of (a) GO and (b) GR.
Figure 7. The formation mechanism of GR by reduction of GO via Hacac.
there are more functional groups in GO than in GR. Moreover, the first drop in the GR curve at 178 °C is attributed to the hydroxyl groups on the basal plane of the GR. It exhibits only a little loss at 254 °C, which is much lower than for GO, indicating a significant decrease in oxygenated functional groups. The later steep drop at 594 °C is caused by the pyrolysis of the remaining oxygen-containing groups as well as the burning of the carbon skeleton. The decomposition temperature of GR (594 °C) is higher than that of GO (538 °C), indicating the removal of the thermally labile oxygen functional groups by chemical reduction, which leads to the enhanced thermal stability of GR. Figure 6 shows the C1s XPS spectra of GO and GR. Before reduction, four different peaks shown in figure 6(a) centered at 284.8, 286.9, 287.8, and 289.1 eV, corresponding to C=C/C-C in aromatic rings, C-O (epoxy and alkoxy), C=O, and COOH groups, respectively, are detected. After the reduction with Hacac and ammonia for 24 h, as shown in figure 6(b), the intensities of all C1s peaks of the carbon binding to oxygen, especially the peak of C-O (epoxy and alkoxy), decreased dramatically, revealing that oxygen-
containing functional groups are removed after reduction [25]. In addition, a new peak at 285.5 eV, corresponding to the C-N group, appears due to the collaborative reduction of Hacac and ammonia [36]. The formation mechanism of GR by reduction of GO via Hacac is shown in figure 7. Epoxy groups of GR are changed into the double bonds, leaving Hacac molecules to increase the stability of GR [32].
3.1. Adsorption of heavy metal ions
Single metal ion equilibrium adsorption studies were conducted to investigate the maximum metal adsorption capacities of carbon nanotubes. The adsorption isotherm of Cd2+ and Co2+ by obtained GR is shown in figure 8(a). It is observed that the amount of adsorbed Cd2+ and Co2+ increases significantly in the range of low equilibrium concentration; it then increases gradually and reaches 51 mg g−1 for Cd2+ and 26 mg g−1 for Co2+ at an equilibrium concentration of 15 mg L−1.
6
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2+
Figure 8. (a) The adsorption isotherm of GR for Cd
and Co2+ at pH 6.0 and (b) the effect of pH on the adsorption of Co2+ and Cd2+ by GR. 2+
Table 1. Langmuir parameters of single adsorption isotherm models
Table 2. Comparison of individual Cd
for Cd
capacities of different adsorbents.
2+
2+
and Co
in GR.
Adsorbent
Cd2+ (mg g−1)
Co2+ (mg g−1)
Graphene
49.28
27.78
Carbon nanotubes Biosorbent
10.86 24.3
— —
Granular AC2
3.37
—
Powdered AC
3.37
—
Titania beans Fe3O4/MnO2 Nanocomposite PHEMA3
8.94 53.2
7.62 —
—
0.74
P(MMA-HEMA)4
—
28.84
Single ion adsorption Types of ions Cd2+ Co2+
qm (mg g−1)
b (L mg−1)
r2
49.28 27.78
7.8 3.1
0.99 902 0.99 578
A linear Langmuir adsorption isotherm model is used to calculate the maximum adsorption amount of Cd2+ and Co2+. The Langmuir absorption equation is as follows:
Ce Ce 1 = + qe bqm qm where Ce stands for the equilibrium concentration of Cd2+ or Co2+ (mg/L), qe is the adsorption amount (mg/g), qm stands for the maximum adsorption capacity corresponding to complete monolayer coverage (mg/g), and b represents the Langmuir constant indirectly related to the energy of adsorption (L/mg). The Langmuir parameters of GR adsorption of Cd2+ and Co2+ are calculated and summarized in table 1. The correlation coefficients, r2, are high and close to 1, suggesting that the experimental data can be represented by the Langmuir adsorption and that the adsorbed metal ions form monolayer coverage on the surfaces of the GR. According to the Langmuir equation, the maximum adsorption qm of Cd2+ is 49.28 mg g−1, the Langmuir constant b equals 7.8, and the correlation coefficient r2 is 0.99 902. Compared with the maximum adsorption of Cd2+ by carbon nanotubes of 10.86 mg g−1, the adsorption capacity of GR is 4.5 times higher [37]. Similarly, the maximum adsorption amount of Co2+ is 27.78 mg g−1, the Langmuir constant b equals 3.1, and the correlation coefficient r2 is 0.99 578. Compared with the maximum adsorption of Co2+ by titania beans of 7.62 mg g−1, the adsorption capacity of GR is 3.6 times higher [38]. The higher adsorption capacity of Cd2+ and Co2+ for as-prepared GR may be attributed to the strong coordination capacity of Hacac and the large surface area of GR in aqueous solutions. The effect of pH on the adsorption of Cd2+ and Co2+ in GR is shown in figure 8(b). When the pH value is lower
and Co2+ adsorption
Conditions
Refs.
pH 6.0; RT1 pH 5.0; RT pH 4.7; 293 K pH 5.0; 303 K pH 5.0; 303 K pH 5.9; RT PH 6.3; RT
our data 37 39
pH 6; 303 K pH 6; 303 K
39 39 38 40 41 41
1
RT stands for room temperature. AC stands for activated carbon. 3 PHEMA stands for poly 2-hydroxyethyl methacrylate. 4 P(MMA-HEMA) stands for copolymer 2-hydroxyethyl methacrylate with monomer methyl methacrylate. 2
than 5.0, the adsorption capacity increases with the increase in pH, which leads mainly to the conclusion that the enol form of Hacac occurs prior to the forming of the coordination complex with Cd2+ and Co2+ in GR. However, the adsorption amount remains almost unchanged as pH values increase from 5.0 to 6.0. Therefore, the pH value 6.0 is chosen as the optimum condition. As the pH values increase from 6.0 to 9.0, the adsorption capacity increases gradually; this may be due to the combined role of adsorption and precipitation. The adsorption capacities of Cd2+ and Co2+ by GR were compared with those of other adsorbents (table 2, with the data collected under different conditions). The Cd2+ adsorption capacity of Fe3O4/MnO2 nanocomposite [40] and the Co2+ adsorption capacity of copolymer 2-hydroxyethyl methacrylate [41] with monomer methyl methacrylate are higher than that of our GR. The metal removal capacities of the other adsorbents such as carbon nanotubes, biosorbent [39], granular 7
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activated carbon (AC), powdered AC, titania beans, and poly 2-hydroxyethyl methacrylate are lower than those of our GR. The comparison suggests that GR has great potential for use in heavy metal ion adsorbents in wastewater treatment.
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4. Conclusions In summary, a green and facile approach to preparing soluble GR has been developed via a new reducing agent, Hacac, which acts as both reducing agent and stabilizer during the formation of GR. Moreover, Hacac is an excellent coordination agent that can coordinate with most metal ions. The maximum adsorption of obtained GR is 49.28 mg g−1 for Cd2+, which is 4.5 times higher than that of carbon nanotubes, and 27.78 mg g−1 for Co2+, which is 3.6 times higher than that of titania beans. The greater capacity of GR to adsorb Cd2+ and Co2+ may be attributed to the strong coordination capacity of Hacac on the surfaces and edges of GR and the large surface area of GR in aqueous solutions. Therefore, we expect that our findings will further the preparation of high-quality GR and GR-based nanocomposites and significantly facilitate the application of GR in water pollution treatment and environmental protection.
Acknowledgments The authors gratefully acknowledge the financial support of the National High-Tech R & D Program of China (863 program, 2011AA050504); the National Natural Science Foundation of China (51102164 and 61376003); the Program for New Century Excellent Talents in University (NCET-120356); Shanghai Science and Technology Grant (12nm0503800), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University.
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