Water-Soluble Phosphated Graphene: Preparation, Characterization

Article pubs.acs.org/IECR

Water-Soluble Phosphated Graphene: Preparation, Characterization, Catalytic Reactivity, and Adsorption Property Hossein Ghafuri* and Majid Talebi Catalyst and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology (IUST), Narmak, Tehran 16846-13114, Iran S Supporting Information *

ABSTRACT: An efficient method for the preparation of water-soluble phosphated graphene for the first time is developed. Graphene oxide (GO) was synthesized through a modified Hummers’ method and functionalized by phosphate groups with phosphorus trichloride and triethylamine in tetrahydrofuran (THF). The morphology and chemical structure of phosphorylated graphene oxide (PGO) and heat-treated PGO (PGO-400) were characterized by X-ray diffraction, Fourier transform infrared spectrometry, scanning electron microscopy, analytical X-ray spectroscopy, atomic force microscopy, diffuse-reflectance spectrometry, thermogravimetric analyses, differential thermogravimetric analysis, Brunauer−Emmett−Teller and Barrett−Joyner−Halenda methods, Raman spectroscopy, and X-ray photoelectron spectroscopy. The acidity of PGO and PGO-400 was measured by a back-titration method. PGO-400 offers extraordinary electronic and thermal properties, cation-exchange capacity, and water dispersibility. The combination of cation-exchange capacity and water dispersibility of PGO-400 offers a variety of applications in organic synthesis and adsorbent sciences. products.11−16 The structure models of GO and functionalized GO have been proposed by numerous researchers. In this regard, Lerf-Klinowski et al. is one of the most prominent groups that illustrated the good capability of GO for functionalization by different chemical reagents. They also had suggested one of the most appropriate structure models for GO in 1998.3−6 Water-soluble graphene (WSG) is one of the most important subtypes of CMG and shows special properties in comparison to GO and reduced graphene oxide (RGO). The synthesized WSG demonstrated high cation-exchange capability and water dispersibility like GO. According to the recorded reports, the synthesized WSG also has properties like RGO such as low oxygen content, high thermal stability, and special electrical properties.17−19 Nowadays, CMG and especially WSG have been used for many applications. Some of these applications are catalytic, biomedical, and electrochemical sensors, adsorption substrates, advanced nanodevices, energy conversion, and energy storage.20−25 On the other hand, various forms of phosphate-containing compounds have been applied in a wide range of applications. Thus, the derivatives of phosphorylated graphene oxide (PGO) as WSGs are useful in a wide range of applications.26−28 The

1. INTRODUCTION It is known that carbon allotropes with nanometric particle dimensions have unusual properties.1 Carbon nanostructures have attracted tremendous research interest in various fields in recent years. Among the inorganic carbon-based materials, graphene oxide (GO) is a two-dimensional carbon nanomaterial that has been emerging as a fascinating material because of its thermodynamic stability, chemical inertness, and oxygencontaining functional groups. These unusual properties have made graphene the most amazing material of the 21th century.2−6 Oxidation of graphite flakes is the best chemical process for the production of GO.7 The Hummers’ method that introduced in 1958 is a well-known procedure for preparation of GO. In the Hummers’ method, the oxidation of graphite was performed by adding potassium permanganate, graphite, and sodium nitrate to a sulfuric acid solution. This method has particular advantages like a feasible approach and a fast synthesis process.8−10 Nowadays, derivatives of graphitic nanostructures can be functionalized by various functional groups, and there are considerable reports about the arrangement, chemical effects, and advantages of functional groups on GO. The electrophilicity of GO is a well-known property that makes a suitable precursor to produce chemically modified graphene (CMG). It is important to select an appropriate method for functionalizing process because of the synthesis procedure effects on the characteristics and applications of functionalized nanostructure © 2016 American Chemical Society

Received: Revised: Accepted: Published: 2970

June 21, 2015 January 11, 2016 February 3, 2016 February 3, 2016 DOI: 10.1021/acs.iecr.5b02250 Ind. Eng. Chem. Res. 2016, 55, 2970−2982

Article

Industrial & Engineering Chemistry Research Arbuzov reaction is a famous method for functionalizing an alkyl bond with a phosphate group. This process takes place by the reaction of trialkylphosphite with an alkyl halide to produce a carbon−phosphate linkage, which causes extension to a polyphosphate graphene (PPG) composite. However, the disadvantage of this extension is the low dispersibility in water.29 To the best of our knowledge, there are not considerable reports for phosphorylation of GO and other graphene-based nanomaterials and for the synthesis of water-soluble phosphated graphene by the present methods. On the basis of these advantages, in this work, we introduce a procedure for the synthesis of water-soluble PGO (WS-PGO) via different chemical and thermal conditions. These phosphorylated products characterized by various methods and also their properties were compared with one another. In addition, some outstanding potential applications of WS-PGO derivatives such as catalysts and adsorbents are also introduced here.

Scheme 1. (a) Reaction Solvents, (b) Phosphorus Reagents, and (c) Catalysts and Additives That Were Tested in the Phosphorylation of GO

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were purchased as analytical grade from Merck and Sigma-Aldrich Company, with no further purification unless otherwise noted. All chemicals and fabrication equipment are listed in Text S1 and S2 in the Supporting Information. 2.2. Methods. 2.2.1. Preparation of GO. GO was prepared according to a modified Hummers’ method.9 In a typical procedure, 4 g of graphite and 2 g of NaNO3 were added to a 500 mL beaker that contained 95 mL of 98% H2SO4. The mixture was stirred for 1 h and then sonicated for 30 min at 65 °C. Then 12 g of KMnO4 was gradually added to the mixture under sonication, while the temperature of the vessel was kept at less than 20 °C. Afterward, the mixture was sonicated at 80 °C for 30 min, stirred for 1 h at 95 °C, poured into 250 mL of deionized (DI) water, and then heated to 100 °C for 1 h (brown solution). When the mixture was poured into 450 mL of DI water, it was diluted. Then H2O2 (5%) was added dropwise until a yellowish-brown solution was obtained. The yellowish mixture was rinsed, repeatedly filtered under vacuum conditions, and rinsed with a solution of HCl (5%) and then with DI water (until the pH increased to 6). The removal of manganese was tested by adding 5 mL of 30% H2O2 to 100 mL of wastewater, and the removal of Cl− was tested by adding 0.1 mL of 0.1 M AgNO3 in 100 mL of wastewater. Finally, the dark-brown powder (GO) was dried in a vacuum oven at 60 °C for 24 h. 2.2.2. Phosphorylation of GO. Phosphorylation of GO for the preparation of WS-PGO was carried out under different reaction conditions. In this regard, the effects of solvents, reagents, catalysts, and additives were evaluated, and the results are shown in Scheme 1. Some protic and aprotic solvents such as DI water, ethanol (EtOH), tetrahydrofuran (THF), dimethylformamide (DMF), diethyl ether (Et2O), and (mono-, di-, tri-, and tetra-) chloromethane (CHxCl4−x) as reaction media were studied.18,30−32 Furthermore, phosphoric acid (H3PO4), phosphoryl chloride (POCl3), phosphorus trichloride (PCl3), phosphorus pentoxide (P2O5), and triethyl phosphite [P(OEt)3] in the same molar ratio were used as phosphorylation reagents.33−35 Also, the catalytic reactivity of some additives was tested in the phosphorylation procedure. Therein, Triethylamine (Et3N), sodium, FeCl3, and MCl (M = H, Na, K) were used as catalysts. 2.2.3. Optimized Method for the Preparation of PGO. The typical procedure for phosphorylation of GO is summarized in

Scheme 2. Note that all steps of this process must be carried out under safe air conditions.36,37 A total of 1 g of synthesized GO was added to a 500 mL two-neck round-bottomed flask containing a mixture of 100 mL of THF and 50 mL of Et3N. The mixture was sonicated for 1 h and cooled to 0 °C. A solution of 5 mL of PCl3 in 100 mL of THF in a dropper funnel was added dropwise under vigorous stirring under controlled temperature, and then the mixture was refluxed in an oil bath for 24 h. After that, the mixture was cooled, cautiously added to 500 mL of DI water, and filtered (GO-PClx), and the waste was collected for recovery of THF and Et3N (introduced in Text S3 in the Supporting Information). The filtrate cake of GO-PClx was dispersed in DI water to form an aqueous mixture. The mixture was neutralized by a NaOH solution (in the pH 8−5 range). Hydrolysis was carried out at 60 °C for 2 h. After that, the mixture was filtered and the powder rinsed with DI water and then EtOH. The wastewater was tested to ensure that the rinsing was complete. The removal of Cl− was checked by adding 0.1 mL of 0.1 M AgNO3 to the 100 mL of wastewater. Furthermore, to investigate the removal of hydrolyzed phosphate, it was checked by adding 0.1 mL of a sodium vanadate−molybdate indicator in 100 mL of wastewater (the preparation of this indicator is explained in Text S4 in the Supporting Information). Finally, the filtrate cake was dried at 80 °C for 12 h. 2.2.4. Optimization of Reaction Scales in the Preparation of PGO. The preparation of PGO was tested in five scales by the same procedure as that mentioned in section 2.2.3. This procedure was done for different amounts of GO from 0.02 to 2971

DOI: 10.1021/acs.iecr.5b02250 Ind. Eng. Chem. Res. 2016, 55, 2970−2982

Article

Industrial & Engineering Chemistry Research

Scheme 2. Preparation Route and Structure Model of PGO, Supposed According to the Lerf and Klinowski Structure Model of GO: (a) GO; (b) GO-PClx; (c) PGO; (d) PGO-400; (e) Hyd-PGO-400

Senterra-2009 spectrometer, equipped with a 785 nm wavelength laser. Wide-angle powder X-ray diffraction (XRD) patterns of the solids were obtained with a JEOL diffractometer with a Cu Kα (λ = 0.15420 nm) X-ray irradiation source in the range of 2θ 5−80°. Scanning electron microscopy (SEM; VEGAII SBU Tescan 2007), using 30 kV in Hi-vac and Au spin coating for sample preparation, was used for surface imaging. Scanning electron microscopy, equipped with energy-dispersive analytical X-ray spectroscopy (EDS), was used for elemental microanalysis. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states of the prepared PGO-400. The data were obtained by using a hemispherical analyzer with an Al Kα X-ray source (hν = 1486.6 eV) operating at a vacuum of better than 10−7 Pa. XPS deconvolution software was utilized to analyze the XPS peaks. Atomic force microscopy (AFM) images were obtained using a DME with a dual-scope C-21 controller and DS 95-50 scanner. Diffuse-reflectance spectrometry (DRS) spectra were collected in the range of 200−800 nm wavelength using a Shimadzo MPC 2200 spectrometer. Thermogravimetric analysis (TGA) and differential thermal analysis (DTG) were performed on a Netzsch TG-209 F1 analyzer in the range of 20− 800 °C and at a heating rate of 10 °C/min. Nitrogen adsorption− desorption isotherms were measured at a liquid-nitrogen temperature of 77 K with a Micromeritcs ASAP 2020 apparatus. The specific surface area was determined by the Brunauer− Emmett−Teller (BET) method. The total pore volume was evaluated by the t-plot method, and the pore-size distribution was analyzed with the supplied Barrett−Joyner−Halenda (BJH) equation software package from the adsorption branches of the isotherms. Four-point-probe (FPP) electrical conductivity of the samples was measured by a Jandel RM3-ar conductometer. A 691 pH meter equipped with an electrode 6.0262.100 of Metrohm Co. was used for titration. 2.3.1. Conductivity Measurements and Sample Fabrication. For facile handling, a flattened (cellulosic) tablet of the samples was used for surface conductivity measurement via FPP analysis. To prepare this tablet, 0.1 g of cellulose fine powder was pressed under 1 Mpa. Then, 1 mg of the sample was rubbed on the cellulose tablet and pressed under 5 Mpa. The morphology of

2 g. The optimized amounts of solvent and reactants in each scale are shown in Table 1. Table 1. Optimized Amounts of Reactants in Five Laboratory Scales for the Preparation of PGO Using PCl3 and Et3N in THF scale GO (g) Et3N (mL) THF (mL)a PCl3 (mL) THF (mL)b flask volume (mL)

1

2

3

4

5

0.02 5 5 1 5−10 25

0.05 5 10 1 10 25

0.1 5−10 20 1−2 20 50

0.5 15−45 50−75 3−7 50−75 150−250

1 25−60 100−150 5−11 100−150 500

Volume of THF in the first mixture (Et3N). bVolume of THF in the second mixture (PCl3). a

2.2.5. Heat Treatment of PGO. As an alternative for hydrolysis and rinsing of GO-PClx, it can be calcined at its critical temperatures. So, GO-PClx was calcined at 250 °C (PGO250) and 400 °C (PGO-400) for 1 h. 2.3. Characterization. The synthesized products were characterized as follows: Fourier transform infrared (FT-IR) spectra were recorded using a Shimadzo 8400 in the range of 350−4700 cm−1, and samples were prepared by a potassium bromide pellet technique. Dispersive Raman spectra were collected in the range of 95−3500 cm−1 (Table 2), by a Bruker Table 2. Raman Scattering Spectroscopy of GO, PGO, PGO250, and PGO-400 D peak (cm‑1) ID (intensity) G peak (cm‑1) IG (intensity) ID/IG

GO

PGO

PGO-250

PGO-400

1304 137.2 1602 96.1 1.43

1298 106.4 1583 56.8 1.87

1292 49.7 1585 27.4 1.81

1301 28.6 1582 16.8 1.70 2972


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Industrial & Engineering Chemistry Research Table 3. Electrical Resistance and Conductivity of Graphite in Comparison to PGO-250 and PGO-400a sample graphite PGO-250 PGO-400

surface resistance (Ω/□) 670 510 95

109 518 99

285 518 97

average resistance (Ω/□)

current (μA)

thickness (cm)

conductivity (S/cm)

516c 97

1 1 1

b ∼7.3 × 10−4 ∼7.0 × 10−4

2.63 14.7

a The wafer thickness (cm), surface resistance (Ω/□), and conductivity (S/cm) are given. bThe wafer thickness varies with the tablet surface. cThe average surface resistance was higher than that of the graphite tablet.

into an Erlenmeyer flask (100 mL). While the mixture was sonicated in an ultrasonic bath, it was degassed by purging with nitrogen under reduced pressure. After 30 min, the ingredients of the mixture were separated by a high-speed centrifuge, the supernatant was poured into an Erlenmeyer flask (100 mL), and for another time, it was degassed. In order to determine the total acidity (end-point) of the mixture, 0.025 M HCl was used as a titrant solution in a buret, and the titration process was surveyed by a pH meter. The back-titration curve of PGO was drawn and the total acidity of the phosphorylated sample calculated by eq 2. The as-prepared solution of 0.020 M Na2CO3 was used to normalize the HCl solution, and the normalized HCl was consumed to standardize the NaOH solution.41,42 eq eq eqPGO = NbVb − NaVa (PGO) = mN g mg (2)

the surface was seen by SEM, and the thickness of the layer was evaluated by a cross-sectional image of the tablet (Figure S1). Afterward, by using a FPP instrument, the surface resistance was collected in Ω/□ units, and this value was converted to S/cm units by the formulas represented in eq 1.16,18 The resistivity and conductivity of the as-prepared tablets are shown in Table 3. sheet resistance = 4.5324(V /I )

(Ω/□)

bulk resistivity of a wafer = sheet resistance × T T = wafer thickness

(cm)

bulk resistivity = 2πs(V /I ) s = probe spacing

(Ω cm)

(cm)

conductance = resistance−1

(Ω cm) where s = 0.1 cm (S) (1)

where V (mL) is the volume of the suspension and m (mg) is the mass of the sorbent. 2.4. Catalytic and Environmental Applications of PGO. The catalytic properties of GO, PGO, and heat-treated PGO were tested in a multicomponent reaction for the synthesis of benzimidazole derivatives. Furthermore, the environmental applications of the samples were examined in dye removal experiments. Thus, samples were used as adsorbents to remove Rhodamine-β (Rh-β) from aqueous media. 2.4.1. General Procedure for Benzimidazole Synthesis. Aldehyde (2.5 mmol), o-phenylenediamine (1 mmol), and 10 mg of PGO were added to a 25 mL round-bottomed flask containing 5 mL of EtOH. The reaction mixture was stirred at room temperature for the times indicated in Table 4. The reaction progress was monitored by thin-layer chromatography by a n-C6H14/EtOAc (3:1) eluent. After completion of the reaction, 20 mL of ethyl acetate was added, and the catalyst was filtered off and used for reusability experiments after rinsing with EtOH. The crude product was obtained after evaporation of the solvent under reduced pressure. Next, the residue was recrystallized with EtOH.43,44 2.4.2. Dye Removal Experiments. A double-beam UV−vis spectrophotometer (PG-LTD-T80 in the range of 190−800 nm wavelength) was used for the dye removal experiments. Dye removal was performed in a 50 mL Erlenmeyer flask containing 25 mL of a dye solution with 20 mg/L Rh-β and 20 mg of sorbent. The Erlenmeyer flask was shaken for 20 min to achieve equilibrium, and then the solvent was separated by centrifugation at 6000 rpm. The supernatant solution was collected by a pipet and poured into a quartz cell. The adsorption efficiency of the dye solution was determined with a UV−vis spectrophotometer before and after the removal process. Experiments were compared for GO, PGO, and PGO-400 as the dye sorbent under ambient conditions. Consolidated residues of the centrifugation process were rinsed with DI water, calcined at 400 °C, and applied to reusability experiments. UV−vis absorbance spectra were collected, and the dye removal efficiencies were calculated using eq 3.45,46

2.3.2. Calculation of the Interlayer Distance between Accumulated PGO Sheets. The local structure of the phosphate functional group was studied by ab initio calculations, and the accumulation of PGO sheets was evaluated by molecular dynamics calculations. These calculations were performed by a pseudopotential plane-wave method within the spin-polarized generalized gradient approximation implemented in the QUANTUM ESPRESSO code.38 Supercells containing 18 carbon atoms and a phosphorus functional group were used in each graphene basal plane. The system was relaxed until the force on each atom was minimized to less than 0.01 eV/Å. Then, accumulation of optimized plates on one another was simulated by molecular dynamics in the two main opposite directions. The minimum interlayer distance was calculated for back-to-face form and in face-to-face directions. 2.3.3. Cation-Exchange Calculation. An amount of 0.50 g of PGO was immersed in 50 mL of 0.025 M NaOH. The mixture was sonicated for 5 min under nitrogen purge conditions. Then, PGO was separated from the mixture using a high-speed centrifuge, while the consolidated residue was obtained without any rinsing with solvents and dried in a vacuum oven at 80 °C for 12 h (Na-PGO). This compound was analyzed with FT-IR spectroscopy, and its spectrum was compared with the PGO spectrum. In addition, the supernatant of the mixture was analyzed by the differential-pH method.39,40 In the same procedure, 0.1 g of PGO-400 was immersed in 25 mL of DI water, and then the containing vessel was sonicated in an ultrasonic bath for 5 min. The hydrolyzed PGO-400 (HydPGO-400) was collected by a high-speed centrifuge, dried in a vacuum oven at 80 °C for 12 h, and analyzed by FT-IR spectroscopy. 2.3.4. Determination of the Total Acidity of the Samples (Back-titration). The total acidity of PGO and PGO-400 was determined by the back-titration method. Note that all steps were done in a nitrogen-filled glovebox and all solutions were degassed by a nitrogen purge. An amount of 0.50 g of PGO was dispersed in 50 mL of 0.025 M NaOH and the mixture poured 2973


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Industrial & Engineering Chemistry Research Table 4. Synthesis of Benzimidazole Derivatives Catalyzed by PGOa

a

Reaction conditions: o-phenylenediamine (1 mmol), aldehyde (2.5 mmol), EtOH solvent (5 mL), and PGO catalyst (10 mg) at room temperature. Products were characterized by comparing their spectroscopic FT-IR and melting point data with those reported in the literature.43,44 cRefer to the reaction yields for the first run with the catalyst (PGO). b

E=

C0 − Ceq C0

× 100

compounds. Also, the types of substituent groups bonding to phosphorus have a wide range, which has a significant effect on the characteristics of the phosphorylated products (Scheme 1b). Phosphoric acid acts as a dehydration reagent and can eliminate most of the functional groups of GO (Figure S2). PCl3 and POCl3 as chlorine phosphorus reagents (PClx) show high reactivity in a substitution reaction, but PClx compounds are strong acids and volatile compounds that pose corrosive and toxic properties. Furthermore, in anhydrous ambient and thermal conditions, “P2O5/EtOH” and “P2O5/MCl (M = H, NH4, Na, K)” produce phosphorus esters [P(OR)x] and PClx, respectively. Thus, these combined reactants were tested to produce in situ active phosphate reagents (Figure S3). These combined reactants have lower hazardous effects than PClx and produce activated phosphorus derivatives simultaneously as the phosphorylation reaction proceeds.33,34,47 Nevertheless, except using the PClx reagents, PPGO has been formed as the major product (Figure S4). According to the mentioned reasons and with utilization of the respective references, PCl3 was chosen as an advantageous reagent for the synthesis of WS-PGO. Also, it has active substituent groups and a low coordination number, which can facilitate the reaction. Acids, bases, metals, and ionic additives can be used as catalysts to facilitate the phosphorylation process. Therein, Et3N, sodium, FeCl3, HCl, and MCl (M = NH4, Na, K) were tested in phosphorylation reactions (Scheme 1c), but most of them have some disadvantages and cannot be used in this reaction. Lewis bases facilitate substituent reactions on PClx and P(OR)x reagents. Lewis acids can activate substituent reactions with P(OR)x reagents and dehydrate GO in the presence of PClx reagents (Figure S5). Ionic additives increase the reactivity of P(OR)x and P2O5/EtOH reagents (Figure S3). a HCl solution

(3)

where E is the adsorption ratio percentage and Qeq (mg/g) is the dye adsorption capacity at equilibrium. C0 (mg/L) and Ceq (mg/ L) represent the initial and equilibrium dye concentrations, respectively.

3. RESULTS AND DISCUSSION 3.1. Phosphorylation of GO. In order to optimize the synthesis conditions of WS-PGO, various experimental conditions were investigated. In some reports, DI water, EtOH, DMF, THF, Et2O, and CHxCl4−x were proposed as suitable solvents for similar reactions (Scheme 1a).18,32 DI water and some of the protic solvents formed a stable dispersion of GO and dissolved the reagents, but solvolysis of phosphate reagents was an important disadvantage of these solvents. This process changes the nature of the reactants and decreases the level of GO−P bonds. The phosphorylation procedure in alcoholic solvents formed polyphosphate graphene oxide (PPGO) composites. These compounds were formed according to the sol−gel procedure. Aprotic solvents such as CHxCl4−x and Et2O dissolve both phosphorus chloride and triethylphosphite, but they were not appropriate solvents for dispersion of GO and products. After several solvents were tested, THF was selected as the reaction solvent because THF has no side reaction with phosphorus reagents and formed a stable dispersion of GO, and its solvent effect produced monophosphated graphene. In some literature, H3PO4, PCl3, POCl3, P2O5, and P(OEt)3 were reported as appropriate phosphorus reagents of the phosphorylation reaction.26−28 Phosphorus has multiple oxidation states and various coordination numbers in different 2974


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Figure 1. FT-IR spectra from top to bottom: graphite; GO; PGO by Et3N and PCl3 in THF (PGO); Na-PGO; PGO-250; PGO-400; Hyd-PGO-400.

compared with those of the non-heat-treated samples. Also, as an alternative of the hydrolysis process and rinsing steps, GO-PClx was calcined at 400 °C. Heat treatment had considerable effects on the thermal stabilization of phosphorus functional groups on phosphorylated GO. It removed all of the volatile impurities that remained in the phosphorylation procedure and decreased the content ratio of oxygen to carbon (O/C) in the heat-treated samples.48,49 3.2. Characterization of GO and PGO Derivatives. To characterize the as-synthesized GO and PGO derivatives, several methods including of FT-IR, Raman, XRD, AFM, SEM, EDS, XPS, TGA, DTG, BET, BJH, and DRS were used. Moreover, the acidic properties and electrical conductivity were evaluated by a back-titration method and FPP measurement, respectively. In FT-IR spectroscopy, the formation of covalent bonds can be confirmed according to the bonding energy and vibration modes of the covalently bonded atoms.50 The existence of oxygen functional groups on GO, the formation of PO, C−O−P, P− O−P, and P−O−H covalent bonds in PGO, and the cationexchange capability of PGO were affirmed by FT-IR spectra. The phosphate functional group has an obvious PO−H stretching peak that appears at around 2800−2900 cm−1. As is shown in Figure 1, PGO (product of the catalytic route with Et3N and PCl3 as phosphate reagents) has a strong witness peak around these wavenumbers. Also, the PO and P−O bonds appear at around 1200−1400 and 900−1100 cm−1, respectively, which depend on the substituents of phosphorus. These ranges of wavenumbers can overlap with the witnesses peaks of GO, so the PO and P− O bonds cannot be clearly specified. Nevertheless, the PGO spectrum can be compared with the spectra of GO, the products of other routes, Na-PGO, heat-treated PGO, and Hyd-PGO-400 samples. In Na-PGO, H+ ions have been exchanged with Na+ ions. Thus, the PO−H bonds have changed to PO−Na bonds, which caused the witness peak of PGO to not be observable around 2900 cm−1. In Raman scattering pattern of graphene-based materials, D band (diamond) indicates the sp3-hybridized carbon and a defected structure in graphitic sheets, and G band (graphite) represents sp2-hybridized carbon and graphitic order. In addition, the intensity ratio of D to G bands is an important factor in determining the graphitic order. The chemical shift of

was not a proper additive for the phosphorylation procedure with any phosphorus reagents (Figure S6). In the case of metal additives such as sodium (Figure S7), although phosphorylation was done, the reduction of oxygen-containing groups of GO was the predominant reaction (Figure S8). After trial and error under different practical conditions, the appropriate phosphorylation procedure was performed via the Ford-Moore and Perry36 and Aoki and Nishio37 procedures, which were reported for the preparation of triethylphosphite and the synthesis of phosphorylated cellulose, respectively. The main procedure involves GO, PCl3, Et3N, THF, NaOH, and DI water, which is schematically represented in Scheme 2. In this method, GO was dispersed in a mixture of THF and Et3N by sonication (first mixture). The containing vessel was kept in an ice bath, and a mixture of PCl3 in THF (second mixture) was dropwisely added to the first mixture under vigorous stirring and temperature control. Blending of these mixtures (PCl3 and Et3N) was critical in this reaction because during this step an endothermic reaction was occurring. Therefore, control of the temperature was critical in the formation of the appropriate product. In order to optimize the scalability of this procedure, the amounts of THF, PCl3, and Et3N compared to GO were evaluated. Then, this procedure was investigated for different amounts of GO. Table 1 shows the optimized amounts of reactants from 0.02 to 1 g of GO in this procedure. The synthesis of PGO was tested for 2 g of GO, but the product had no preferable properties such as good dispersibility and homogeneity. Afterward, the mixture was refluxed for 24 h at 80 °C. It was cooled to room temperature, poured cautiously into DI water, and hydrolyzed by a NaOH solution. At the end of this step, all of the P−Cl bonds of GO-PClx were converted to P−OH covalent bonds. After that, the mixture was filtered, and the contaminants were rinsed with EtOH and DI water. The product was dried and weighed, and in the case of using 1 g of GO, 0.86 g of PGO was obtained. PGO had critical temperatures in the thermal analysis that were suitable for thermal treatments. Then, PGO was calcined at 250 and 400 °C, which were higher than its critical temperatures. Heat-treated PGO samples were weighed and used for further analysis. The structures and properties of these samples were 2975


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Figure 2. (a) Raman scattering patterns of GO, PGO, PGO-250, and PGO-400. (b) XRD patterns of graphite, GO, PGO-250, and PGO-400.

Figure 3. EDS of (a) GO, (b) phosphorylated GO by POCl3 (PGO−POCl3), (c) heat-treated PGO−PCl3 at 250 °C (PGO-250), (d) heat-treated PGO−PCl3 at 400 °C (PGO-400), (e) PPGO by P2O5/EtOH, KCl (PPGO), and (f) heat-treated PPGO at 400 °C by P2O5/EtOH, NaCl route (PPGO-400).

into randomly oriented sheets. In the case of PGO-250, a broad weak peak appeared at around 21°. With an increase in the calcination temperature in PGO-400, its peak shifted to upper angles and became more intense. EDS is a valuable technique that gives both qualitative and quantitative elemental analysis data. EDS spectra of GO and phosphorylated samples are depicted in Figure 3. EDS analysis of GO confirmed the presence of oxygen and carbon (with about 1:3 O/C ratio), and so EDS analysis of PGO indicated the existence of phosphorus in the structures of the samples. It can be considered that the result of the phosphorylation process was a decrease of the relative oxygen content of samples and also, by calcination of the PGO derivatives, the O/C peak ratios significantly decreased.54 Moreover, most of the impurities such as chlorine and sulfur were removed by thermal decomposition. SEM images show the effect of each process on the morphology of products. The conversion of oxygen-containing carbon to a phosphorylated carbon caused disorderness in the basal plane, and this was shown by the XRD patterns and Raman spectra previously. Accordingly, this viewpoint can be confirmed by comparing the SEM images in Figure 4a,b. Furthermore, the substitution reactivities of PCl3 and POCl3 were compared with the SEM images in Figure 4b,d, alongside the FT-IR (Figure S4) and EDS analysis (Figure 3).

Raman peaks is an indirect evidence for the accumulation of graphene layers on one another.51 Raman scattering patterns of GO and phosphorylated samples are shown in Figure 2a. During the oxidation of graphite, the graphitic order of the graphene planes was decreased, and at the same time, the dispersibility was increased. Also, during phosphorylation of GO, the structural order of the planes was decreased. On the other hand, with increasing calcination temperature, the graphitic order was increased. In the case of PGO-400, a weak 2D band shows low layer-by-layer accumulation of sheets. (The G band/D band intensity ratio data are shown in Table 2.) Figure 2b shows the XRD patterns of the pristine graphite, GO, PGO, PGO-250, and PGO-400 samples. The interlayer distances in the graphite and GO were calculated with the peak position in the XRD patterns and using the Bragg equation (nλ = 2d sin θ).52,53 Graphite and GO have shown (001) diffraction peaks around (2θ) 26.5° and 11.6°, which were equal to 0.35 and 0.77 nm for their interlayer distances, respectively. This change was due to the formation of oxygen-containing functional groups on the carbon basal planes. This result is in agreement with Raman and FT-IR analysis and can prove the preparation of GO. After functionalization of GO with the phosphate group, the (001) peak vanished and PGO did not show any significant diffraction peaks. This result indicates the amorphous structure for PGO and confirms that the PGO sheets were fully exfoliated 2976


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Figure 4. SEM images of (a) GO and phosphorylated GO samples in Et3N, THF procedure, (b) PCl3 (PGO), (c) heat-treated PGO (PGO-400), (d) POCl3 (PGO−POCl3), (e) PPGO by P2O5/EtOH (PPGO), and (f) heat-treated PPGO (PPGO-400).

Figure 5. AFM image of PGO-400. The sample was coated on a mica substrate and analyzed through tapping mode: (a) perspective (3D); (b) top view (2D); (c) height profile of the line pathway.

stretching band in FT-IR analysis at around 1600 cm−1, the G/D intensity ratios in the Raman scattering pattern, and the EDS elemental analysis and SEM images of PGO with PGO-400 (Figure 4b,c). The electrical conductivity of PGO-400 in comparison to its raw materials was significantly increased. The results of FPP measurements are summarized in Table 3. As can be concluded from conductivity measurements, after thermal treatment, the length of the conjugated system and the electron free path were increased in the basal planes. The radius and valence of phosphorus atoms are different from those of carbon atoms. Despite this fact, doping of phosphorus in the graphene defects is one of the possible ways for graphene− phosphorus reactions. Doping in basal planes forms a phosphorus-containing bridge between carbon atoms of PGO derivatives. The doping process can be affected by the type of

The formation of PPGO led to swollen layers that were observed in the SEM images. In the presence of P2O5/EtOH as a combined phosphate reagent, the structure of PPGO formed and was confirmed in the FT-IR spectrum with an intense peak near 900 cm−1 (Figure S4). Also, this is obvious in the EDS (Figure 3e,f) and SEM (Figure 4a,e) images. In addition, by calcination of PPGO at 400 °C (PPGO-400), a high-order intercalated PPG composite was formed. PPGO-400 has a stable structure and does not show good dispersibility in water, and its polyphosphate scaffold shows a water-swollen property with excellent degradation resistance in dilute acidic and alkaline media (Figure S9). By calcination of the phosphorylated samples, the graphitic order increased and the signs of the oxygen functional groups vanished. This interpretation was confirmed by the CC 2977


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factors. The first factor is the reduction of the O/C content in the PGO structure, which was confirmed by elemental analysis, and the second is the protection effect of the phosphorus-containing groups that were bonded to PGO. After phosphorylation of GO, a lot of surface wrinkles were observed on its basal planes. These types of surface wrinkles create submicron and nanosize porous structures and generate PGO as a high-surface-area substrate. Nevertheless, PGO has an aggregated structure in the solid state because the phosphorylated sheets have been affected by strong hydrogen bonding. By calcination of PGO, its hydrate content was decreased. Thus, the dehydrated PGO was considered to be a low-density material. The surface properties of PGO-250 and PGO-400 were analyzed by BET and BJH methods. Nitrogen adsorption−desorption isotherms and distribution of the pore diameters can be seen in Figure 8. The results indicate that PGO-400 has a greater capacity in the nitrogen adsorption−desorption isotherm. The calculated BET surface areas of PGO-250 and PGO-400 were 12.8 and 41.3 m2/g, respectively. These nanomaterials have a wide range of porous structures. Probably, these porous structures were formed by wrapping and twisting of the accumulated sheets. Their nitrogen adsorption isotherms did not exactly match with the standard isotherms. Thus, their pore-size distribution varied between the meso- and macropore range. Nevertheless, the PGO-400 isotherm is similar to the type IV isotherm, which refers to a mesoporous material. The optical properties of samples in the UV−vis range were compared using DRS analysis. Figure 9a shows the DRS spectra of GO, PGO, and PGO-400. After functionalization of GO, which looks dark brown, PGO becomes black and has a strong absorption in the visible and near-IR. When the DRS spectra of PGO-400 were compared to PGO, a further red shift to visible and near-IR regions was shown. This can be caused by changes in the sp3-hybridized to conjugated sp2-hybridized carbon atoms over the graphene basal planes. Commonly, solid acids with an equinormal amount of phosphoric groups have shown more acidic properties than similar substrates with carboxylic groups. In order to demonstrate the cation exchange between 0.50 g of PGO and 50 mL of a 0.022 M NaOH solution, the pH of the supernatant of the solution was surveyed. Hereupon, its pH decreased from 12.3 to 11.5; therefore, the pH of the solution has fallen a 0.8 unit. For cation exchange in DI water, the same process was done in quantities of 50, 100, and 1000 mL of DI water. In these cases, the initial pH was 6.6, and the ultimate values reached 4.0, 4.2, and 5.8.39,57

phosphorus precursor, conditions of the substitution reaction, and heat treatment procedure.49,55 Deconvoluted XPS analysis of PGO-400 confirms that about 82% of the carbon atoms have a conjugated sp2 hybrid, less than 9% of the carbon atoms were directly bonded to phosphorus atoms, and about 9% of the carbon atoms were directly bonded to oxygen atoms (Figure S10). The phosphorylation procedure was the dominant reaction in comparison to phosphorus doping, because just after 5 min of sonication, PGO and PGO-400 formed a homogeneous and stable suspension in aqueous media. This dispersibility was attributed to hydrogen bonding between the phosphate functional groups with water molecules (Table S1 and Figures S11 and S12). A dilute colloid of PGO-400 (5 mg/L) was coated on a mica substrate, and its height profile was analyzed by a noncontact AFM method. Figure 5 shows the surface roughness and existence of the sheets with a uniform thickness of about 1− 1.5 nm. Thus, PGO-400 can be considered to be a WS-PGO. The results of the AFM height profiles have good agreement with the computational results of the PGO interlayer distances. In this case, the structure and accumulation of PGO sheets were optimized by ab initio and molecular dynamics theories, respectively. Accordingly, the minimum interlayer distances of the back-to-face and face-to-face forms were calculated as 5.59 and 9.82 Å, respectively (Figure 6). These results can be affirmed by the reported literature.56

Figure 6. Graphical view of layer-by-layer accumulation and interlayer distances (Å) of PGO sheets. Calculated by ab initio and molecular dynamics theory in the (a) face-to-face and (b) back-to-face directions.

The thermal behavior and thermal stability of pristine GO and PGO were analyzed by means of the TGA and DTG methods. The TGA and DTG curves in the range of 20−800 °C are depicted in Figure 7. As can be seen in the TGA curves, GO shows a higher mass loss rate than PGO. In addition, it has lower critical temperatures in the DTG profile. Hence, during heat treatment, PGO has better thermal decomposition resistance than GO. These observations are associated with two significant

Figure 7. (a) TGA and (b) DTG curves of GO and PGO. 2978


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Figure 8. (a) Nitrogen adsorption−desorption isotherm. (b) Pore diameters of (1) PGO-250 and (2) PGO-400.

Figure 9. (a) DRS spectra of GO, PGO, and PGO-400. (b) Back-titration curve of PGO; procedure surveyed by a pH meter (solution pH versus VolHCl).

Back-titration is one of the accurate methods for measurement of the total acidity of the substrates.41,42 Thus, a mixture of PGO and NaOH solution was homogenized in an ultrasonic bath, and the supernatant of the mixture was separated by a high-speed centrifuge. Then, the supernatant was titerd by a standard HCl solution. The titration curve of PGO is depicted in Figure 9b, and the total acidity of PGO was calculated by eq 2. Therein, the total acidities of PGO and PGO-400 were 1.63 and 1.72 mequiv/g, respectively. 3.3. Evaluation of the Applications of PGO. The benzimidazole synthesis is a multicomponent reaction in organic chemistry. Benzimidazole synthesis has two- and threecomponent products, depending on the type of catalyst (Scheme 3a). The three-component benzimidazole is the predominant product in the presence of phosphorus-containing substrates.43,44 This reaction was tested in the presence of PGO and showed acceptable results in the synthesis of threecomponent products. The results of the catalytic reactivities of PGO are summarized in Table 4. In addition, the reusability of the PGO nanocatalyst four times in the synthesis of 1-(3nitrobenzyl)-2-(3-nitrophenyl)-1H-benzimidazole (Table 4, entry 2) is represented in Figure 10. Another advantage of our synthesized nanocatalyst (PGO) is very good selectivity in comparison to those of GO and functionalized GO in this reaction. While in the presence of GO and some of the functionalized GO, the outcome of this reaction includes a mixture of products, PGO only generates one product in the synthesis of benzimidazole. Besides, some of the functional groups of GO can be reacted with the organic moieties of this reaction.58,59 Thus, in the benzimidazole synthesis, GO has not

Scheme 3. (a) General Reaction for the Synthesis of Three-/ Two-Component Benzimidazole and (b) the Supposed Mechanism for the Three-Component Synthesis of Benzimidazole Using PGO

shown selectivity and reusability in comparison to PGO (Scheme 3b). Also, phosphorylated products have potential applications for several usages in new fields such as recyclable adsorbents of environmental pollutants from aqueous media.60,61 Thus, GO, PGO, and PGO-400 were tested for the removal of Rh−B from aqueous media (Figure 11a,b). In the first run of dye removal experiments, GO, PGO, and PGO-400 adsorbed 94%, 53%, and 99% of Rh−B, respectively. PGO-400 has edge functional groups that cause its high 2979



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02250. Four texts that describe the list of materials, synthesis and fabrication equipment, recovery of the waste of the phosphorylation reaction, preparation of the phosphate indicator, a table of concentrations, and 12 figures that show SEM, FT-IR, EDS, XPS, AFM, concentration profiles, and optical images associated with the context (PDF)

Figure 10. Reaction yields for the synthesis of entry 2 in Table 4 [(3nitrobenzyl)-2-(3-nitrophenyl)-1H-benzimidazole] after recovery of the catalyst. Reaction conditions: o-phenylenediamine (1 mmol), aldehyde (2.5 mmol), EtOH solvent (5 mL), and PGO catalyst (10 mg for the first run) at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +98-21-77240516-7. Fax: +98-21-77491204. E-mail: [email protected].

dispersibility in water. Also, it can adsorb Rh−B in the basal plane by high π−π interactions between the aromatic moieties of dye and sp2-hybridized carbon atoms of the planes.62−64 In addition, the thermal stability of PGO-400 facilitated its thermal remediation and made it a convenient adsorbent for reusability experiments, while GO cannot be thoroughly recovered via thermal remediation (Figure 11c).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge partial support from the Research Council of IUST.

4. CONCLUSIONS Phosphorylation of GO and modification procedures with different chemical and thermal conditions were discussed in this paper. Functionalized products were characterized by various methods, and an appropriate procedure was introduced for the preparation of WS-PGO. PGO and heat-treated PGO showed acidic properties in aqueous solutions. Also, heat-treated PGO showed high surface area, special electrical conductivity, and good dispersibility in aqueous media. Therefore, heat-treated PGO can be considered to be a WSG. Therein, PGO derivatives with excellent advantages can be used as nanocatalysts in new fields of green organic synthesis and renewable and recyclable adsorbents for the removal of pollutants.

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Figure 11. (a) UV−vis spectra and (b) optical images of Rh−B removal experiments using 20 mg of GO, PGO, and PGO-400 adsorbents from 25 mL of a 20 mg/L stock solution. (c) Dye removal efficiencies of adsorbents after thermal recovery at 400 °C. 2980


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