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Modifying graphite oxide with grafted methyl acrylate brushes for the attachment of magnetite nanoparticles
Applied Surface Science 280 (2013) 235–239
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Modifying graphite oxide with grafted methyl acrylate brushes for the attachment of magnetite nanoparticles Xiang Liu ∗ , Heming Cheng, Yan Liu School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, China
a r t i c l e
i n f o
Article history: Received 17 February 2013 Received in revised form 25 April 2013 Accepted 27 April 2013 Available online 7 May 2013 Keywords: Graphite oxide Polymerization Magnetite Catalysis
a b s t r a c t Surface-grafted polymer brushes of methyl acrylate of graphite oxide (GO) have been constructed to anchor poly(amidoamine) 2.0G (PAMAM 2.0G) dendrites for the attachment of magnetite nanoparticles. Monomers of methyl acrylate were copolymerized on the GO sheets by atom transfer radical polymerization. Then the PAMAM 2.0G was attached via covalent bond at the end of the grafted chains. Thus the inexpensive Fe(III) of ferric trichloride could coordinate with the remaining amino groups of PAMAM 2.0G and were reduced by the added sodium borohydrite at appropriate pH value to obtain nano magnetitedecorated GO hybrid materials. The hybrids exhibited a powerful catalysis on the degradation of aqueous solutions of hydroquinone. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphite oxide (GO) is an interesting material due to its novel structure of pseudo-2D with large amount of active groups such as hydroxyl, carboxyl and epoxy which generated in the course of the preparation. These functional groups are capable of involving many sorts of reactions [1–9] which are utilized as effective avenues to realize some proper modifications on GO. Such chemical ornaments give rise to wider applications of GO [3,4,10]. The hydroxyls of GO reacted with N-(trimethoxysilylpropyl) ethylenediamine triacetic acid to introduce a multi-carboxyl species onto GO and thus stable suspensions were obtained in ethanol/Nafion solutions [10]. Such mixtures were deposited on a glass carbon electrode forming a stable film which displayed high ion selectivity in an electrochemical catalysis. The hydroxyls of GO could also be activated by 2-bromo-2-methylpropanoyl bromide (BMPB) to introduce the initiators onto its surface leading to surface-grafted copolymerization of methacrylates by technique of atom transfer radical polymerization (ATRP). After reduced with hydrazine hydrate the polymer brush-modified GO enhanced piezoelectric -polymorph poly(vinylidine fluoride) formation [4]. The side carboxyls of GO could be esterified by N-hydroxysuccinimide and subsequently reacted with ethylenediamine so as to introduce primary amino groups. Hence the initiator molecules such as BMPB could be attached to surface of GO by amidation reaction. Consequently, monomer of 2-(dimethylamino)ethyl methacrylates
∗ Corresponding author. Tel.: +86 555 2311551. E-mail address: [email protected] (X. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.141
were graftedly copolymerized on GO [5]. Additionally, the surface functional groups of - –OH or–COOH of GO were utilized to anchor Mg/Ti catalyst species giving a nanoscale catalyst system of synthesizing polypropylene on GO [3]. Treatments on GO with isocyanates brought an enhanced stability in polar aprotic solvents such as N,N-dimethylformamide (DMF), N-methylpyrrolidone, dimethyl sulfoxide (DMSO) which resulted from the reaction between hydroxyls and isocyano groups [7]. Apparently, many of these novel applications of GO were derived from the appropriate modifications on GO chemically. However, some precursors with polar carbonyls, such as ferric triacetylacetonate [11,12] or ferric pentacarbonyl [13], could interact with unmodified GO by hydrogen bonds after a thorough dispersion in supersonic dealing. Then the mixtures were heated to a high temperature to obtain nano magnetite/GO composites which exhibited a surprising magnetoresistance [13] and peroxidase-like activity [14]. The nano magnetite possesses the catalysis on degradation of p-nitrophenol based on the chemical activities and the large specific surface area [15]. This property was utilized to deal with environmental polluters such as phenolic wastewater. Despite the unmodified GO sheets possess the capability of loading nano magnetite [11,13,16], which based on the direct interactions between the precursors and surface functional groups of GO, we believe that the appropriate modifications on GO will capture more precursors of nanoparticles and improve their loading capabilities. PAMAM dendrites with plenty of outside amino groups had been applied in preparing metallic nanoparticles by the coordination of amino groups with corresponding ions by means of in situ reducing reaction in the presence of reductants. The PAMAM 2.0G
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Fig. 1. The stepwise modifications on GO were described schematically. The initiated GO (represented with GO–BMPB) was obtained by reaction (i) in the first place. Then MA were graftedly copolymerized on GO (abbreviated with GO–BMPB–MA) by means of ATRP technique (ii). The end esters of the grafted polymer brushes reacted with PAMAM 2.0G by virtue of amidation reaction (iii) to obtain the composites of GO–BMPB–MA–PAMAM. Thus the Fe(III) were captured and were reduced through reaction (iv) under appropriate conditions to generate magnetite-decorated GO hybrid materials (denoted with GO–BMPB–MA–PAMAM–Fe).
molecules can link with GO by covalent bonds as long as the GO is mended properly. Such PAMAM/GO hybrids will increase the loading of Fe(III). Hence ferric trichloride (FeCl3 ) rather than ferric triacetylacetonate or ferric pentacarbonyl can be chosen as an inexpensive Fe(III) source in preparing magnetite/GO hybrids. This approach will be described in this work in detail. The whole procedure is depicted schematically in Fig. 1.
2. Experimental 2.1. Materials Ammonia (30%), tetrafuran (THF), hydrogen peroxide (H2 O2 , 30%), methyl acrylate (MA), sodium borohydrite (NaBH4 ), FeCl3 , DMF and DMSO were purchased from Guoyao Chemicals (Shanghai, China). BMPB, cuprous chloride (CuCl), cupric chloride (CuCl2 ), N,N ,N ,N -pentamethyldiethylenetriamine (PMDETA) and hydroquinone were commercially provided by Xiya Chemicals (Chengdu, China). Solvents of THF were dehydrated with anhydrous calcium chloride before use. Other chemicals were used as received without any purification. GO was prepared by Hummer’s method using graphite flakes oxided with H2 SO4 and KMnO4 [17]. PAMAM 2.0G was synthesized by successive addition reaction between ethylenediamine and MA as well as amidation between the resultant esters and ethylenediamine which reported elsewhere [18].
2.2. Preparation of grafted polymer brushes on GO surface About 100 mg GO and 20 mL THF were mixed by ultrasonication for 30 min. Then the mixtures were transferred into a reactor followed with dropwise addition of 1.5 mL BMPB depicted in (i) of Fig. 1. Such reaction system was oil-sealed to hinder ambient water vapors and lasted for 5 h at 35 ◦ C with a magnetic agitation. The products, GO–BMPB, were obtained by vacuum filtration with three times washing of anhydrous alcohol and subsequently dried at 50 ◦ C. About 25 mg GO–BMPB mixed with 20 mL DMSO under supersonic disposal for 30 min. Subsequently, 3 mL MA, 100 mg CuCl (1.01 mmol), 50 mg CuCl2 (0.372 mmol) and 0.1 mL PMDETA were added to the GO–BMPB/DMSO suspensions. The reaction system was airtight with an oil-sealed device and was kept under flowing N2 for at least 10 min so as to vent the air. Such reaction (ii, depicted in Fig. 1) proceeded for 6 h at 60 ◦ C. The products (GO–BMPB–MA) were filtrated and washed with copious water for many times and anhydrous alcohol for three times.
2.3. Attachment of PAMAM 2.0G to the side chains of grafted polymer brushes of GO–BMPB–MA and generation of nano magnetite About 25 mg GO–BMPB–MA were dissolved in 20 mL DMSO with supersonic disposal for 30 min followed with dropwise
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Fig. 2. IR spectra of GO and GO–BMPB corresponding with (a) and (b) respectively. Regions of 1859–1508 cm−1 and 1508–879 cm−1 are magnified and shown in insets to have a clear observation.
addition of 5 mL PAMAM 2.0G/DMSO solutions (10 g/mL). The reaction (iii in Fig. 1) was controlled at 40 ◦ C for 4.5 h and the products (GO–BMPB–MA–PAMAM) were achieved by filtration with a thorough washing of a lot of hot water to remove any unreacted PAMAM 2.0G and dried at 50 ◦ C. Weighed 25 mg GO–BMPB–MA–PAMAM and mixed with 20 mL DMF with a supersonic disposal for 10 min. 5 mL 0.5 M FeCl3 was added and the pH value of the mixtures was modulated at 9.0 with 5% ammonia. Next, 50 mg NaBH4 was put into the reaction system little by little with a vigorous stirring at first. Then the reaction (iv in Fig. 1) lasted for 3 h at 60 ◦ C with a smooth agitation. The products (GO–BMPB–MA–PAMAM–Fe) were filtrated with many times washing of copious water to remove any inorganic impurity. 2.4. Catalysis of the nano magnetite–GO hybrids toward the degradation of hydroquinone About 10 mg of GO–BMPB–MA–PAMAM–Fe was dispersed supersonically in 20 mL water followed with addition of 10 mL 1 mM hydroquinone solutions. The pH value of such mixtures was adjusted at 9.0. Then 5 mL 30% H2 O2 was added the system with a gentle agitation. The degradative progress of hydroquinone was traced with ultraviolet–visible (UV–vis) spectra. 2.5. Characterization techniques The stepwise modifications on GO were measured with infrared (IR) spectroscopy (Nicolet 380, Thermo Nicolet Corporation). The introducing of PAMAM 2.0G onto the terminals of grafted polymer brushes was monitored with a UV–vis spectroscopy (specord 200, Analytik Jena AG). Nanoparticles loaded on modified GO were observed with a transmission electron microscope (TEM) (H7650, Hitachi). The GO and GO–BMPB–MA–PAMAM–Fe hybrids were measured with thermogravimetric analyses (TGA) conducted on Thermax 700 (Thermo Fisher Scientific) with nitrogen purge gas and a heating rate of 10 ◦ C/min. The samples were grinded to homogeneous powders to ensure excellent heat diffusions. 3. Results and discussion 3.1. Introduction of initiators on GO surface During the preparation of GO, graphite was exfoliated into sheets by strong oxidants of KMnO4 and concentrated H2 SO4 leading to a large amount of hydroxyls at the edge and basal planes. The strong and wide band centered at 3397 cm−1 shown in Fig. 2(a)
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Fig. 3. IR spectra of (a), (b) and (c) correspond with hybrids of GO–BMPB–MA, GO–BMPB–MA–PAMAM and GO–BMPB–MA–PAMAM–Fe respectively.
illuminates stretching vibration of v(O H) which derived from OH and COOH [19,20]. Peaks of 1721 and 1624 cm−1 should be ascribed to stretching vibration of v(C O) of carboxyls and v(C C) of aromatic skeletons of GO respectively [7,21–23]. The epoxy groups of GO are determined with a faint peak of 1226 cm−1 which is shown in the right insert [21,24]. GO can be initiated with BMPB resulting in esters which illuminated in IR spectrum of Fig. 2(b). The intensity of v(O H) depresses obviously which caused by the esterification between OH of GO and BMPB. The resultant esters are determined by v(C O) of 1733 cm−1 displayed in enlarged ranges of the left insert. The nearby 1721 cm−1 is remained which shows that the COOH is intact during such modifications. Besides, the new peaks of 948 and 1003 cm−1 exhibited in the right insert should be assigned to the coupling of scissoring vibrations of C H of the two methyls which bonded with the tertiary carbon in GO–BMPB. On account of the strong electronegativity of bromine atom, both the two peaks have a blue-shift.
3.2. Attachment of PAMAM 2.0G to surface grafted polymer brushes of GO Monomers of MA are graftedly copolymerized on the initiated GO giving GO–BMPB–MA hybrids which is determined by IR spectrum of Fig. 3(a). A broad band nearby 1716 cm−1 should belong to stretching vibration of v(C O) coming from overlapping of two different ester groups. The terminal esters of the side chain of the grafted polymer brushes react with PAMAM 2.0G by amidation reaction. Thus the broad band near 1716 cm−1 vanishes obviously which claims that the esters of the side chains are changed into amides shown in Fig. 3(b). Amide I and II as well as v(C C) and ı(N H) of amino groups overlap to form a broad band between 1485 and 1684 cm−1 as shown in Fig. 3(b). Additionally, the alternations of UV–vis spectra respond such chemical modifications since both GO and PAMAM 2.0G have the characteristics of UV–vis response. Fig. 4(a) demonstrates the obtained GO. The typical band 230 nm is attributed to –* transition of C C of aromatics and a shoulder peak of 300 nm is assigned to n–p* transition of C O [21]. Band 281 nm is the characteristic of PAMAM 2.0G [18,25,26] which is shown in Fig. 4(c). The broad band ranged from 250 to 275 nm shown in Fig. 4(b) demonstrates the attachment of PAMAM 2.0G to the surface grafted polymer brushes of GO covalently. The shoulder peak of 230 nm still belongs to C C of aromatics of GO. The anchored PAMAM 2.0G provides dense amino groups which are liable to coordinate with metallic ions. Such coordinated metallic ions may be reduced into metallic nanoparticles [27].
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Fig. 4. UV–vis spectra of (a), (b) and (c) correspond GO–BMPB–MA–PAMAM hybrids and PAMAM 2.0G respectively.
with
GO,
3.3. Generation of nano magnetite on surface of GO and its catalysis toward the degradation of hydroquinone When Fe(III) mixed with the GO–BMPB–MA–PAMAM composites in DMF solutions some of them coordinated with primary amino groups of the composites while the others were reduced into Fe(II) in the presence of NaBH4 . Then magnetite particles were manufactured at pH 9.0 which is described in the following reaction equation. Fe(III) + 2Fe(II) + 8OH − = Fe3 O4 + 4H2 O The formation of magnetite particles are judged from IR spectra of Fig. 3(c). The intensive peaks of 630 and 557 cm−1 [11] along with surface plasma spectrum as shown in Fig. 5(a) at 378 nm witness the generation of nano magnetite on GO–BMPB–MA–PAMAM surface. The PAMAM 2.0G may provide a beneficial environment which is preferable for growth of nano magnetite. In order to demonstrate this we arrange a control experiment. In the experiment, the unmodified GO mixed with Fe(III) followed with addition of NaBH4 . The obtained product gives UV–vis spectrum as shown in Fig. 5(b). An extremely faint peak near 378 nm demonstrates that unmodified GO is unfavorable for preparing nano magnetite under such conditions. This fact illuminates that introducing of PAMAM 2.0G onto GO benefits the generation of nano magnetite. PAMAM 2.0G provides an excellent template for anchoring metallic ions
Fig. 5. GO–BMPB–MA–PAMAM hybrids and GO are mixed with Fe(III) respectively which reduced with NaBH4 under the same conditions giving respective UV–vis spectra in (a) and (b).
Fig. 6. TEM image of GO–BMPB–MA–PAMAM–Fe hybrids shown in (a); such hybrids are dispersed in water which attracted by a magnet shown in (b).
on account of its plentiful amino groups. Additionally, GO are reduced simultaneously by the added NaBH4 which is judged from the red shift of –* transition from 230 nm shown in Fig. 4(a) to 254 nm [28] shown in Fig. 5(a). TEM image of Fig. 6(a) exhibits a large amount of nanoparticles on the surfaces of GO. Such GO–BMPB–MA–PAMAM–Fe hybrids disperse well in water and are distinguishable when placing a magnet outside the suspensions which is shown in Fig. 6(b). All the hybrid materials in the water are collected tidily. Such solid hybrids in bottle can stand up when placing a magnet outside the bottom of the bottle and can rotate when placing it on a magnetic stirrer which is switched on. These confirm their strong magnetism. Fig. 7 displays the magnetization hysteresis loops of the GO–BMPB–MA–PAMAM–Fe hybrids which are measured at 300 K in the magnetic field between −20,000 and 20,000 Oe. It is obvious that the sample exhibits superparamagnetism and nearly no remnant magnetization is observed. TGA curve of GO illuminates that GO have a mass loss of 15% between 200 and 250 ◦ C which is shown in Fig. 8(a). This may be ascribed to loss of absorbed small molecules such as water and decomposition of the peripheral oxygen-containing functional groups of GO [6]. On account of the stacking of GO sheets, the oxygen-containing functional groups on basal planes are hard to be pyrolyzed. Such functional groups interact by means of van der Walls forces or hydrogen bonding and thereby they are stabilized. There is a sudden drop between 600 and 750 ◦ C with a rough weight loss of 25% which shows the removal of such more stable functional groups. This tells us that more functional groups are generated on
Fig. 7. The magnetization hysteresis loops of GO–BMPB–MA–PAMAM–Fe hybrid at 300 K.
X. Liu et al. / Applied Surface Science 280 (2013) 235–239
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4. Summary Monomers of MA are graftedly copolymerized on surfaceinitiated GO by ATRP technique. The terminal esters of the side chains react with primary amino groups of PAMAM 2.0G dendrites to introduce dense amino groups onto the GO surface which capture a large amount of Fe(III) in FeCl3 solutions. Thus nano magnetite–GO hybrid materials are manufactured by virtue of reducing reactions under appropriate conditions. This strategy provides a new way of modifying GO chemically to construct a preferable environment of preparing magnetite-fabricated GO hybrid materials. The inexpensive metallic iron and organic monomers as well as gentle preparing conditions facilitate the manufacture of such hybrids. It is believed that such easy avenues of modifications on GO will promote wider applications of GO materials such as in treating the environmental contaminants. Fig. 8. TGA curves of GO and GO–BMPB–MA–PAMAM–Fe hybrids corresponding with (a) and (b) respectively.
Acknowledgement This work was supported by Educational Commission of Anhui province of China (KJ2012A051). References
Fig. 9. UV–vis spectra of tracing the degradation progress of hydroquinone aqueous solutions in the presence of H2 O2 catalyzed by hybrids of GO–BMPB–MA–PAMAM–Fe (pH 9.0, room temperature).
the basal planes of GO than that on the edges during the oxidation of graphite [9,11,29]. Fig. 8(b) displays a slow weight loss before 500 ◦ C due to the degradation of the grafted polymer brushes and the anchored PAMAM dendrites. The most organic species of GO decompose when temperature reaches 600 ◦ C [9,11]. The GO–BMPB–MA–PAMAM–Fe hybrids exhibit catalysis on degradation of hydroquinone in the presence of H2 O2 at pH 9.0 and room temperature. In Fig. 9, band 288 nm drops rapidly within only 10 min. The hydroquinones are degraded entirely after 60 min which demonstrates that the nano magnetite loaded on GO possesses a strong catalysis on decomposition of hydroquinone. This arises from the size effect of nano magnetite which increases the catalytic sites and well dispersion of the nanoparticles which is promoted by the furnished GO. Nano magnetites with mixed-valence react with H2 O2 bringing radicals of • OH and • OOH [30] which catalyze the decomposition of hydroquinones [15]. The conductive property of graphene facilitates the electronic transport and changes of the valences, which are helpful for exerting of the catalysis. Hence, such GO–BMPB–MA–PAMAM–Fe hybrids may have a potential applications in treating environmental contaminants.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Modifying graphite oxide with grafted methyl acrylate brushes for the attachment of magnetite nanoparticles Xiang Liu ∗ , Heming Cheng, Yan Liu School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, China
a r t i c l e
i n f o
Article history: Received 17 February 2013 Received in revised form 25 April 2013 Accepted 27 April 2013 Available online 7 May 2013 Keywords: Graphite oxide Polymerization Magnetite Catalysis
a b s t r a c t Surface-grafted polymer brushes of methyl acrylate of graphite oxide (GO) have been constructed to anchor poly(amidoamine) 2.0G (PAMAM 2.0G) dendrites for the attachment of magnetite nanoparticles. Monomers of methyl acrylate were copolymerized on the GO sheets by atom transfer radical polymerization. Then the PAMAM 2.0G was attached via covalent bond at the end of the grafted chains. Thus the inexpensive Fe(III) of ferric trichloride could coordinate with the remaining amino groups of PAMAM 2.0G and were reduced by the added sodium borohydrite at appropriate pH value to obtain nano magnetitedecorated GO hybrid materials. The hybrids exhibited a powerful catalysis on the degradation of aqueous solutions of hydroquinone. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Graphite oxide (GO) is an interesting material due to its novel structure of pseudo-2D with large amount of active groups such as hydroxyl, carboxyl and epoxy which generated in the course of the preparation. These functional groups are capable of involving many sorts of reactions [1–9] which are utilized as effective avenues to realize some proper modifications on GO. Such chemical ornaments give rise to wider applications of GO [3,4,10]. The hydroxyls of GO reacted with N-(trimethoxysilylpropyl) ethylenediamine triacetic acid to introduce a multi-carboxyl species onto GO and thus stable suspensions were obtained in ethanol/Nafion solutions [10]. Such mixtures were deposited on a glass carbon electrode forming a stable film which displayed high ion selectivity in an electrochemical catalysis. The hydroxyls of GO could also be activated by 2-bromo-2-methylpropanoyl bromide (BMPB) to introduce the initiators onto its surface leading to surface-grafted copolymerization of methacrylates by technique of atom transfer radical polymerization (ATRP). After reduced with hydrazine hydrate the polymer brush-modified GO enhanced piezoelectric -polymorph poly(vinylidine fluoride) formation [4]. The side carboxyls of GO could be esterified by N-hydroxysuccinimide and subsequently reacted with ethylenediamine so as to introduce primary amino groups. Hence the initiator molecules such as BMPB could be attached to surface of GO by amidation reaction. Consequently, monomer of 2-(dimethylamino)ethyl methacrylates
∗ Corresponding author. Tel.: +86 555 2311551. E-mail address: [email protected] (X. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.141
were graftedly copolymerized on GO [5]. Additionally, the surface functional groups of - –OH or–COOH of GO were utilized to anchor Mg/Ti catalyst species giving a nanoscale catalyst system of synthesizing polypropylene on GO [3]. Treatments on GO with isocyanates brought an enhanced stability in polar aprotic solvents such as N,N-dimethylformamide (DMF), N-methylpyrrolidone, dimethyl sulfoxide (DMSO) which resulted from the reaction between hydroxyls and isocyano groups [7]. Apparently, many of these novel applications of GO were derived from the appropriate modifications on GO chemically. However, some precursors with polar carbonyls, such as ferric triacetylacetonate [11,12] or ferric pentacarbonyl [13], could interact with unmodified GO by hydrogen bonds after a thorough dispersion in supersonic dealing. Then the mixtures were heated to a high temperature to obtain nano magnetite/GO composites which exhibited a surprising magnetoresistance [13] and peroxidase-like activity [14]. The nano magnetite possesses the catalysis on degradation of p-nitrophenol based on the chemical activities and the large specific surface area [15]. This property was utilized to deal with environmental polluters such as phenolic wastewater. Despite the unmodified GO sheets possess the capability of loading nano magnetite [11,13,16], which based on the direct interactions between the precursors and surface functional groups of GO, we believe that the appropriate modifications on GO will capture more precursors of nanoparticles and improve their loading capabilities. PAMAM dendrites with plenty of outside amino groups had been applied in preparing metallic nanoparticles by the coordination of amino groups with corresponding ions by means of in situ reducing reaction in the presence of reductants. The PAMAM 2.0G
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Fig. 1. The stepwise modifications on GO were described schematically. The initiated GO (represented with GO–BMPB) was obtained by reaction (i) in the first place. Then MA were graftedly copolymerized on GO (abbreviated with GO–BMPB–MA) by means of ATRP technique (ii). The end esters of the grafted polymer brushes reacted with PAMAM 2.0G by virtue of amidation reaction (iii) to obtain the composites of GO–BMPB–MA–PAMAM. Thus the Fe(III) were captured and were reduced through reaction (iv) under appropriate conditions to generate magnetite-decorated GO hybrid materials (denoted with GO–BMPB–MA–PAMAM–Fe).
molecules can link with GO by covalent bonds as long as the GO is mended properly. Such PAMAM/GO hybrids will increase the loading of Fe(III). Hence ferric trichloride (FeCl3 ) rather than ferric triacetylacetonate or ferric pentacarbonyl can be chosen as an inexpensive Fe(III) source in preparing magnetite/GO hybrids. This approach will be described in this work in detail. The whole procedure is depicted schematically in Fig. 1.
2. Experimental 2.1. Materials Ammonia (30%), tetrafuran (THF), hydrogen peroxide (H2 O2 , 30%), methyl acrylate (MA), sodium borohydrite (NaBH4 ), FeCl3 , DMF and DMSO were purchased from Guoyao Chemicals (Shanghai, China). BMPB, cuprous chloride (CuCl), cupric chloride (CuCl2 ), N,N ,N ,N -pentamethyldiethylenetriamine (PMDETA) and hydroquinone were commercially provided by Xiya Chemicals (Chengdu, China). Solvents of THF were dehydrated with anhydrous calcium chloride before use. Other chemicals were used as received without any purification. GO was prepared by Hummer’s method using graphite flakes oxided with H2 SO4 and KMnO4 [17]. PAMAM 2.0G was synthesized by successive addition reaction between ethylenediamine and MA as well as amidation between the resultant esters and ethylenediamine which reported elsewhere [18].
2.2. Preparation of grafted polymer brushes on GO surface About 100 mg GO and 20 mL THF were mixed by ultrasonication for 30 min. Then the mixtures were transferred into a reactor followed with dropwise addition of 1.5 mL BMPB depicted in (i) of Fig. 1. Such reaction system was oil-sealed to hinder ambient water vapors and lasted for 5 h at 35 ◦ C with a magnetic agitation. The products, GO–BMPB, were obtained by vacuum filtration with three times washing of anhydrous alcohol and subsequently dried at 50 ◦ C. About 25 mg GO–BMPB mixed with 20 mL DMSO under supersonic disposal for 30 min. Subsequently, 3 mL MA, 100 mg CuCl (1.01 mmol), 50 mg CuCl2 (0.372 mmol) and 0.1 mL PMDETA were added to the GO–BMPB/DMSO suspensions. The reaction system was airtight with an oil-sealed device and was kept under flowing N2 for at least 10 min so as to vent the air. Such reaction (ii, depicted in Fig. 1) proceeded for 6 h at 60 ◦ C. The products (GO–BMPB–MA) were filtrated and washed with copious water for many times and anhydrous alcohol for three times.
2.3. Attachment of PAMAM 2.0G to the side chains of grafted polymer brushes of GO–BMPB–MA and generation of nano magnetite About 25 mg GO–BMPB–MA were dissolved in 20 mL DMSO with supersonic disposal for 30 min followed with dropwise
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Fig. 2. IR spectra of GO and GO–BMPB corresponding with (a) and (b) respectively. Regions of 1859–1508 cm−1 and 1508–879 cm−1 are magnified and shown in insets to have a clear observation.
addition of 5 mL PAMAM 2.0G/DMSO solutions (10 g/mL). The reaction (iii in Fig. 1) was controlled at 40 ◦ C for 4.5 h and the products (GO–BMPB–MA–PAMAM) were achieved by filtration with a thorough washing of a lot of hot water to remove any unreacted PAMAM 2.0G and dried at 50 ◦ C. Weighed 25 mg GO–BMPB–MA–PAMAM and mixed with 20 mL DMF with a supersonic disposal for 10 min. 5 mL 0.5 M FeCl3 was added and the pH value of the mixtures was modulated at 9.0 with 5% ammonia. Next, 50 mg NaBH4 was put into the reaction system little by little with a vigorous stirring at first. Then the reaction (iv in Fig. 1) lasted for 3 h at 60 ◦ C with a smooth agitation. The products (GO–BMPB–MA–PAMAM–Fe) were filtrated with many times washing of copious water to remove any inorganic impurity. 2.4. Catalysis of the nano magnetite–GO hybrids toward the degradation of hydroquinone About 10 mg of GO–BMPB–MA–PAMAM–Fe was dispersed supersonically in 20 mL water followed with addition of 10 mL 1 mM hydroquinone solutions. The pH value of such mixtures was adjusted at 9.0. Then 5 mL 30% H2 O2 was added the system with a gentle agitation. The degradative progress of hydroquinone was traced with ultraviolet–visible (UV–vis) spectra. 2.5. Characterization techniques The stepwise modifications on GO were measured with infrared (IR) spectroscopy (Nicolet 380, Thermo Nicolet Corporation). The introducing of PAMAM 2.0G onto the terminals of grafted polymer brushes was monitored with a UV–vis spectroscopy (specord 200, Analytik Jena AG). Nanoparticles loaded on modified GO were observed with a transmission electron microscope (TEM) (H7650, Hitachi). The GO and GO–BMPB–MA–PAMAM–Fe hybrids were measured with thermogravimetric analyses (TGA) conducted on Thermax 700 (Thermo Fisher Scientific) with nitrogen purge gas and a heating rate of 10 ◦ C/min. The samples were grinded to homogeneous powders to ensure excellent heat diffusions. 3. Results and discussion 3.1. Introduction of initiators on GO surface During the preparation of GO, graphite was exfoliated into sheets by strong oxidants of KMnO4 and concentrated H2 SO4 leading to a large amount of hydroxyls at the edge and basal planes. The strong and wide band centered at 3397 cm−1 shown in Fig. 2(a)
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Fig. 3. IR spectra of (a), (b) and (c) correspond with hybrids of GO–BMPB–MA, GO–BMPB–MA–PAMAM and GO–BMPB–MA–PAMAM–Fe respectively.
illuminates stretching vibration of v(O H) which derived from OH and COOH [19,20]. Peaks of 1721 and 1624 cm−1 should be ascribed to stretching vibration of v(C O) of carboxyls and v(C C) of aromatic skeletons of GO respectively [7,21–23]. The epoxy groups of GO are determined with a faint peak of 1226 cm−1 which is shown in the right insert [21,24]. GO can be initiated with BMPB resulting in esters which illuminated in IR spectrum of Fig. 2(b). The intensity of v(O H) depresses obviously which caused by the esterification between OH of GO and BMPB. The resultant esters are determined by v(C O) of 1733 cm−1 displayed in enlarged ranges of the left insert. The nearby 1721 cm−1 is remained which shows that the COOH is intact during such modifications. Besides, the new peaks of 948 and 1003 cm−1 exhibited in the right insert should be assigned to the coupling of scissoring vibrations of C H of the two methyls which bonded with the tertiary carbon in GO–BMPB. On account of the strong electronegativity of bromine atom, both the two peaks have a blue-shift.
3.2. Attachment of PAMAM 2.0G to surface grafted polymer brushes of GO Monomers of MA are graftedly copolymerized on the initiated GO giving GO–BMPB–MA hybrids which is determined by IR spectrum of Fig. 3(a). A broad band nearby 1716 cm−1 should belong to stretching vibration of v(C O) coming from overlapping of two different ester groups. The terminal esters of the side chain of the grafted polymer brushes react with PAMAM 2.0G by amidation reaction. Thus the broad band near 1716 cm−1 vanishes obviously which claims that the esters of the side chains are changed into amides shown in Fig. 3(b). Amide I and II as well as v(C C) and ı(N H) of amino groups overlap to form a broad band between 1485 and 1684 cm−1 as shown in Fig. 3(b). Additionally, the alternations of UV–vis spectra respond such chemical modifications since both GO and PAMAM 2.0G have the characteristics of UV–vis response. Fig. 4(a) demonstrates the obtained GO. The typical band 230 nm is attributed to –* transition of C C of aromatics and a shoulder peak of 300 nm is assigned to n–p* transition of C O [21]. Band 281 nm is the characteristic of PAMAM 2.0G [18,25,26] which is shown in Fig. 4(c). The broad band ranged from 250 to 275 nm shown in Fig. 4(b) demonstrates the attachment of PAMAM 2.0G to the surface grafted polymer brushes of GO covalently. The shoulder peak of 230 nm still belongs to C C of aromatics of GO. The anchored PAMAM 2.0G provides dense amino groups which are liable to coordinate with metallic ions. Such coordinated metallic ions may be reduced into metallic nanoparticles [27].
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Fig. 4. UV–vis spectra of (a), (b) and (c) correspond GO–BMPB–MA–PAMAM hybrids and PAMAM 2.0G respectively.
with
GO,
3.3. Generation of nano magnetite on surface of GO and its catalysis toward the degradation of hydroquinone When Fe(III) mixed with the GO–BMPB–MA–PAMAM composites in DMF solutions some of them coordinated with primary amino groups of the composites while the others were reduced into Fe(II) in the presence of NaBH4 . Then magnetite particles were manufactured at pH 9.0 which is described in the following reaction equation. Fe(III) + 2Fe(II) + 8OH − = Fe3 O4 + 4H2 O The formation of magnetite particles are judged from IR spectra of Fig. 3(c). The intensive peaks of 630 and 557 cm−1 [11] along with surface plasma spectrum as shown in Fig. 5(a) at 378 nm witness the generation of nano magnetite on GO–BMPB–MA–PAMAM surface. The PAMAM 2.0G may provide a beneficial environment which is preferable for growth of nano magnetite. In order to demonstrate this we arrange a control experiment. In the experiment, the unmodified GO mixed with Fe(III) followed with addition of NaBH4 . The obtained product gives UV–vis spectrum as shown in Fig. 5(b). An extremely faint peak near 378 nm demonstrates that unmodified GO is unfavorable for preparing nano magnetite under such conditions. This fact illuminates that introducing of PAMAM 2.0G onto GO benefits the generation of nano magnetite. PAMAM 2.0G provides an excellent template for anchoring metallic ions
Fig. 5. GO–BMPB–MA–PAMAM hybrids and GO are mixed with Fe(III) respectively which reduced with NaBH4 under the same conditions giving respective UV–vis spectra in (a) and (b).
Fig. 6. TEM image of GO–BMPB–MA–PAMAM–Fe hybrids shown in (a); such hybrids are dispersed in water which attracted by a magnet shown in (b).
on account of its plentiful amino groups. Additionally, GO are reduced simultaneously by the added NaBH4 which is judged from the red shift of –* transition from 230 nm shown in Fig. 4(a) to 254 nm [28] shown in Fig. 5(a). TEM image of Fig. 6(a) exhibits a large amount of nanoparticles on the surfaces of GO. Such GO–BMPB–MA–PAMAM–Fe hybrids disperse well in water and are distinguishable when placing a magnet outside the suspensions which is shown in Fig. 6(b). All the hybrid materials in the water are collected tidily. Such solid hybrids in bottle can stand up when placing a magnet outside the bottom of the bottle and can rotate when placing it on a magnetic stirrer which is switched on. These confirm their strong magnetism. Fig. 7 displays the magnetization hysteresis loops of the GO–BMPB–MA–PAMAM–Fe hybrids which are measured at 300 K in the magnetic field between −20,000 and 20,000 Oe. It is obvious that the sample exhibits superparamagnetism and nearly no remnant magnetization is observed. TGA curve of GO illuminates that GO have a mass loss of 15% between 200 and 250 ◦ C which is shown in Fig. 8(a). This may be ascribed to loss of absorbed small molecules such as water and decomposition of the peripheral oxygen-containing functional groups of GO [6]. On account of the stacking of GO sheets, the oxygen-containing functional groups on basal planes are hard to be pyrolyzed. Such functional groups interact by means of van der Walls forces or hydrogen bonding and thereby they are stabilized. There is a sudden drop between 600 and 750 ◦ C with a rough weight loss of 25% which shows the removal of such more stable functional groups. This tells us that more functional groups are generated on
Fig. 7. The magnetization hysteresis loops of GO–BMPB–MA–PAMAM–Fe hybrid at 300 K.
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4. Summary Monomers of MA are graftedly copolymerized on surfaceinitiated GO by ATRP technique. The terminal esters of the side chains react with primary amino groups of PAMAM 2.0G dendrites to introduce dense amino groups onto the GO surface which capture a large amount of Fe(III) in FeCl3 solutions. Thus nano magnetite–GO hybrid materials are manufactured by virtue of reducing reactions under appropriate conditions. This strategy provides a new way of modifying GO chemically to construct a preferable environment of preparing magnetite-fabricated GO hybrid materials. The inexpensive metallic iron and organic monomers as well as gentle preparing conditions facilitate the manufacture of such hybrids. It is believed that such easy avenues of modifications on GO will promote wider applications of GO materials such as in treating the environmental contaminants. Fig. 8. TGA curves of GO and GO–BMPB–MA–PAMAM–Fe hybrids corresponding with (a) and (b) respectively.
Acknowledgement This work was supported by Educational Commission of Anhui province of China (KJ2012A051). References
Fig. 9. UV–vis spectra of tracing the degradation progress of hydroquinone aqueous solutions in the presence of H2 O2 catalyzed by hybrids of GO–BMPB–MA–PAMAM–Fe (pH 9.0, room temperature).
the basal planes of GO than that on the edges during the oxidation of graphite [9,11,29]. Fig. 8(b) displays a slow weight loss before 500 ◦ C due to the degradation of the grafted polymer brushes and the anchored PAMAM dendrites. The most organic species of GO decompose when temperature reaches 600 ◦ C [9,11]. The GO–BMPB–MA–PAMAM–Fe hybrids exhibit catalysis on degradation of hydroquinone in the presence of H2 O2 at pH 9.0 and room temperature. In Fig. 9, band 288 nm drops rapidly within only 10 min. The hydroquinones are degraded entirely after 60 min which demonstrates that the nano magnetite loaded on GO possesses a strong catalysis on decomposition of hydroquinone. This arises from the size effect of nano magnetite which increases the catalytic sites and well dispersion of the nanoparticles which is promoted by the furnished GO. Nano magnetites with mixed-valence react with H2 O2 bringing radicals of • OH and • OOH [30] which catalyze the decomposition of hydroquinones [15]. The conductive property of graphene facilitates the electronic transport and changes of the valences, which are helpful for exerting of the catalysis. Hence, such GO–BMPB–MA–PAMAM–Fe hybrids may have a potential applications in treating environmental contaminants.
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