A structural study on ethylenediamine- and poly(amidoamine)-functionalized graphene oxide: simultaneous reduction, functionalization, and formation of 3D structure

Abstract

Graphene oxide (GO) nanosheets were simultaneously modified and reduced with ethylenediamine (EDA). Then, EDA-treated GO (RGO-NH₂) was exposed to iterative alkylation and amidation reactions to synthesize poly(amidoamine) (PAMAM)-grafted GO nanosheets. Reactions were continued until 4th generation PAMAM was grafted onto GO layers (RGO-4.0GD). FT-IR results confirmed the synthesis of the different structures, while TGA curves showed that the reactions that occurred were non-ideal and PAMAM was not grafted as expected. Also, XRD patterns showed that there is a resistance against the intercalation of nanolayers named stitching. According to the Raman spectra, the modification progression resulted in a more disordered structure of the graphene layers due to grafting PAMAM generations whereas reduction occurred continuously. Morphological studies were performed by recording SEM and TEM images. The results showed that the treated-GO nanosheets re-stack together, while distinguishable edges of nanolayers were observed.

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Introduction

In the recent years, graphene as a one-atom-thick planar nanosheet has received much attention due to its remarkable properties such as superior mechanical properties,¹ excellent electronic transport properties,² etc. It is widely used in electronic devices,³ drug delivery systems,⁴ preparation of nanocomposites,⁵ etc. However, in most applications, graphene is modified with different molecules to overcome its agglomeration or to reach better dispersion.^6,7 The most useful method to produce modified-graphene is oxidation of graphite to graphene oxide (GO), subsequent modification of GO with modifiers,^8,9 and finally reduction of the remaining oxygenated groups.¹⁰ Although this method is one of the promising ways to produce modified-GO on a large scale, reduction of the remaining oxygen-containing groups is necessary to improve electrical and thermal properties.^11,12 This can be performed via thermal shocking at elevated temperature,¹³ electrochemical,¹⁴ and chemical reduction.¹⁵ However, reduction of GO results in agglomeration of nanosheets. Simultaneous functionalization during reduction can be an effective method to prevent re-stacking of graphene layers.^16,17 In this field, ethylenediamine (EDA) as a small-molecule-modifier acts as a reducing agent to deoxygenate GO;^18,19 it can remove hydroxyl and epoxide moieties or convert epoxides to amino alcohol.²⁰ It also reacts with acid moieties on the edge of GO nanosheets. EDA-modified GO (RGO-NH₂) contains amine groups that can be used in different modification processes such as amidation,²¹ Michael addition,²² etc. So, RGO-NH₂ can act as a precursor to synthesize a poly(amidoamine) dendritic structure which is used in drug delivery systems and biological applications.^23,24 Therefore, successful simultaneous reduction and modification of GO with ethylenediamine may open new horizons for the application of GO in biomedical applications. In this work, ethylenediamine was used as simultaneous reducing agent and modifier of GO. The synthesized RGO-NH₂ was used as a multifunctional core to synthesize PAMAM-grafted RGO nanosheets. Fourier transform infrared (FT-IR) and Raman spectroscopy were used to characterize different bands in the produced materials. The thermal behaviour of the samples was investigated via thermal gravimetric analysis (TGA) and X-ray diffraction (XRD); scanning electron microscope (SEM) and transmission electron microscope (TEM) were used for a structural study.

Experimental section

Materials

Methyl acrylate (Sigma-Aldrich, 99%, MA) was passed through a basic alumina column to remove the inhibitor. Graphite fine powder (Merck, extra pure), sodium nitrate (Fluka, >99%, NaNO₃), sulphuric acid (Merck, 98%), potassium permanganate (Merck, 99%), hydrogen peroxide (Mojallali, 35%, H₂O₂), hydrochloric acid (Mojallali, aqueous solution, 37%), N,N′-dicyclohexylcarbodiimide (Aldrich, 99%, DCC), 4-dimethylaminopyridine (Aldrich, 99%, DMAP), n-hexane (Merck, >99%), ethanol (Merck, 99.9%), methanol (Merck, 99.9%) and ethylenediamine (Merck, ≥99%, EDA) were used as received.

Synthesis of GO from graphite

GO was prepared using a modified Hummers’ method according to a previously described procedure.²⁵ Briefly, NaNO₃ (1.000 g) and graphite (2.000 g) were dispersed in H₂SO₄ (180 mL) and the mixture was stirred for 15 min at room temperature. Then, KMnO₄ (6.000 g) was slowly added into the mixture and the temperature was kept under 20 °C. After raising the temperature to 35 °C, the mixture was stirred for 7 h. Subsequently, KMnO₄ (6.000 g) was added and stirring was continued for an additional 12 h. Then, the mixture was diluted with deionized water (400 mL) and H₂O₂ (20 mL, 30%) was added to reduce the unreacted KMnO₄. Oxidized graphite was obtained via centrifugation, washing with hydrochloric acid solution (1/10 with respect to water), and washing with distilled water to reach neutral pH. The obtained powder was exfoliated by water bath ultrasonication for 1 h (0.1 mg mL⁻¹). After filtration, GO powder was dried under vacuum at 60 °C.

Synthesis of RGO-NH₂

Dried GO powder (2.000 g) was dispersed in ethanol (100 mL) via ultrasonication for 30 min and ethylenediamine (5 mL) was added. Then, DCC (6.190 g) and DMAP (0.620 g) were added to the mixture and the reaction was continued at 70 °C for 72 h. RGO-NH₂ was precipitated with addition of n-hexane (300 mL) and centrifuged at 10 000 rpm. After precipitation of the product in n-hexane (100 mL) three times, RGO-NH₂ was dried under vacuum at 50 °C overnight.

Grafting of PAMAM dendritic structures onto RGO-NH₂

To synthesize the PAMAM dendritic structure, a previously described method was used.²⁶ To this end, a divergent method with iterative sequence Michael addition or alkylation and amidation reactions with RGO-NH₂ as primary core and methyl acrylate was used. Initially, a Michael addition reaction was performed between peripheral amine groups of RGO-NH₂ (2.000 g) and methyl acrylate (5 mL) in methanol (150 mL) at room temperature for 4 days. The mixture was centrifuged at 10 000 rpm, washed with ethanol several times and dried in vacuum at 50 °C overnight. The reaction of the obtained product (1.500 g) with ethylenediamine (12 mL) via an amidation reaction led to the synthesis of RGO-1.0GD. The reaction was performed in methanol (150 mL) at room temperature for 5 days. The mixture was centrifuged at 10 000 rpm, washed with ethanol several times and dried under vacuum at 50 °C overnight. This iterative sequence of reactions was continued under nitrogen atmosphere and dark room conditions at room temperature until RGO-4.0GD was synthesized. It is noteworthy that EDA and MA were doubled in each generation progression. The expected reaction mechanism is depicted in Scheme 1.

Scheme 1 Expected mechanism of PAMAM grafting onto GO nanosheets.

Instrumentation

Fourier transform infrared (FT-IR) spectroscopy was performed by means of a Bruker Tensor 27 FT-IR-spectrophotometer, in the range between 500 and 4000 cm⁻¹ with a resolution of 4 cm⁻¹. An average of 24 scans has been carried out for each sample. The samples were prepared on a KBr pellet in vacuum desiccators under a pressure of 0.01 Torr. Thermal gravimetric analyses were carried out by means of a PL thermo-gravimetric analyser (Polymer Laboratories, TGA 1000, UK). All samples (about 10 mg) were heated from ambient temperature to 600 °C at a heating rate of 10 °C min⁻¹ and nitrogen as the purging gas was used at a flow rate of 50 mL min⁻¹. X-ray diffraction (XRD) spectra were performed on an X-ray diffraction instrument (Siemens D5000) with a Cu target (λ = 0.1540 nm) at room temperature. The system consists of a rotating anode generator which operated at 35 kV and 20 mA. The samples were scanned from 2θ = 2 to 80° in the step scan mode. The diffraction pattern was collected using a scintillation counter detector. Raman spectroscopy was performed in the range from 200 to 3000 cm⁻¹ using a Bruker Dispersive Raman Spectrometer fitted with a 785 nm laser source, a CCD detector, and a confocal depth resolution of 2 μm. The laser beam was focused on the sample using an optical microscope. A Vega Tescan SEM analyser (Czech Republic) was used to evaluate the morphology of the neat and modified graphenes which were gold-coated using a sputtering coater. The specimens were prepared by coating a thin layer on a mica surface using a spin coater (Modern Technology Development Institute, Iran). A transmission electron microscope (TEM), Tescan Mira, with an accelerating voltage of 120 kV was used to study the morphology of the samples. All the samples were prepared by a drop-dry method on carbon-coated copper grids.

Results and discussion

The synthesis of GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD were monitored by means of FT-IR as depicted in Fig. 1. In the G spectrum, the strong characteristic peak at 3430 cm⁻¹ is related to the water molecules absorbed by the sample or KBr powder.²⁷ The stretching vibrations of aromatic C=C bonds are observed at 1640 and 1575 cm⁻¹.²⁸ After the oxidation process, four kinds of oxygen-containing moieties including epoxide (C–O–C), hydroxyl (–OH), carbonyl (–C=O), and carboxyl (–COOH) are created in the GO where as investigated previously, epoxide and hydroxyl are located on the basal plane of GO as the major types and carbonyl and carboxyl exist at the edges as the minor types.²⁰ As a result, the GO FT-IR spectrum shows several characteristic peaks of oxygenated groups comprising absorptions at 1049 cm⁻¹ (C–O–C stretching vibrations),²⁹ 1223 cm⁻¹ (C–OH stretching vibrations),³⁰ 1406 cm⁻¹ (O–H deformation of the C–OH groups) and 1724 cm⁻¹ (C=O stretching vibrations of the –COOH groups).³¹ As previously reported, amine-containing compounds such as hydrazine³² and ethylenediamine¹⁹ act as reducing agents to deoxygenate GO. Such compounds can remove hydroxyl and epoxide moieties or convert epoxides to hydrazino/amino alcohols.²⁰ However, ethylenediamine can react with carboxylic groups which are not removed by means of deoxygenation. As a result of functionalization with ethylenediamine, the peak of the C=O stretching vibration in the GO structure at 1720 cm⁻¹ shifts to a lower position and coincides with the C=C stretching vibration. So, no distinct peak is observed for C=O in RGO-NH₂ except at 1640 cm⁻¹. Also, the epoxide-originated peak at 1049 cm⁻¹ disappears and a new characteristic peak at 1085 cm⁻¹ appears due to the C–N stretching of amine-containing groups.³³ The absorption peak at 1218 cm⁻¹ is ascribed to hydroxyl groups at the edge of an aromatic domain.²⁰ Medium/weak-intensity peaks in the 1300–1460 cm⁻¹ region are ascribed to the antisymmetric C–N stretching vibrations coupled with the out-of-plane NH₂ and NH modes.³⁴ The peaks at 2850 and 2925 cm⁻¹ are also attributed to the stretching vibration of C–H bonds introduced by ethylenediamine molecules. Also, the peak at 3321 cm⁻¹ belongs to N–H stretching vibrations. Further modification with PAMAM hyper-branched molecules alters the FT-IR spectra of the resulting structures where a merged bimodal absorption peak with peak values of 1620 and 1640 cm⁻¹ appears in the range of 1550–1750 cm⁻¹. Also, the peaks at 2850 and 2925 cm⁻¹ attributed to the C–H bonds become stronger with modification progression and N–H stretching vibrations shift to 3292 cm⁻¹. Also, the characteristic peak at around 3400–3500 cm⁻¹ comes apart and transforms to two bands due to primary amines of PAMAM attached molecules.

Fig. 1 FTIR spectra of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD.

For further proving the success of the modification process, all samples were analysed by means of TGA. TGA and DTG curves of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD are depicted in Fig. 2 and 3 respectively. Also, TGA results are summarized in Table 1. The mass loss of all samples around 100 °C is ascribed to the loss of moisture and adsorbed water molecules. The π-stacked structure of GO results in the storage of some water molecules and a mass loss of about 7.3 wt% is observed until 110 °C.³⁵ According to the results, G shows no significant weight loss up to 600 °C. However, GO shows further two main degradation stages between 110–600 °C. A major weight loss between 110 and 210 °C that corresponds to the weight loss of ∼30 wt% is related to CO, CO₂, and H₂O released from the most labile functional groups.³⁶ A slower mass loss between 210 and 600 °C is attributed to the degradation of more stable oxygen functionalities and corresponds to a weight loss of ∼18 wt%.³⁷ As reported previously,³⁸ ethylenediamine simultaneously reduces, functionalizes and stitches GO nanosheets. Stitching originated from the presence of two amine (–NH₂) functionalities on both sides of the ethylene moiety. When the two –NH₂ functional groups attack the epoxide carbon/carboxylic moieties of two different GO sheets, it helps to form a stitched structure. According to the results, after reaction of GO with ethylenediamine, the storage of water molecules in the π-stacked structure is reduced heavily and a mass loss of ∼4.6 wt% is observed. Also, the second stage of degradation shifts from 165 to 196.3 °C with weight loss of ∼12 wt%. This shows that ethylenediamine significantly reduces the oxygen-containing moieties of GO nanosheets or reacts with labile groups that leads to their higher thermal stability.³⁹ Also, instead of a degradation stage at 266.3 °C, due to degradation of amine groups,³⁴ a sharp degradation is observed at 279.6 °C which continues with a slower degradation with T_d,max of 311.7 °C. This is attributed to the degradation of ethylene moieties covalently attached to the surfaces of nanosheets. Assuming the third stage of degradation is attributed to the functionalized moieties, more weight loss is expected with functionalization progression and attachment of more hyper-branched PAMAM. However, in RGO-1.0GD, the third stage of degradation shows a little increase in weight loss with respect to RGO-NH₂ and the weight loss increases from 15.98 to 18.78 wt%. This shows that reacted EDA exists in the form of primary amine-containing groups but incompletely i.e. some ethylene moieties do not contain primary amines to participate in chain extension. This originates from the reaction of both amine groups of EDA with two different GO nanosheets which is reported by Lee et al.³⁸ as stitching. It is also noteworthy that after attaching PAMAM onto the nanosheets, the thermal stability above 260 °C is improved and the mass loss increases while in RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD samples, T_d,max increases to 387.9, 400.0, 410.7, and 418.2 °C respectively. This is attributed to the attachment of more PAMAM on the surface of nanosheets which improves thermal stability due to the existence of amide moieties.⁴⁰ In this regard, the mass loss increases to 18.8, 22.3, 29.4 and 37.5 wt% for RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD respectively which shows that functionalization takes place from non-stitching moieties. Also, more functionalization leads to a greater decrease in mass loss at the second stage which shows some of the ethylenediamine molecules are consumed as reducing agent of oxygen-containing groups in each modification step.

Fig. 2 TGA curves of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD.

Fig. 3 DTG curves of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD.

Table 1 Summarized results of TGA for G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD

Sample	Stage 1		Stage 2		Stage 3
	Td,max (°C)	Weight loss (%)	Td,max (°C)	Weight loss (%)	Td,max (°C)	Weight loss (%)
G	25–130 °C		130–600 °C		—
	107.0	0.01	—	0.25	—	—
GO	25–110 °C		110–210 °C		210–600 °C
	74.0	7.35	165.0	30.09	266.3	17.90
RGO-NH2	25–130 °C		130–260 °C		260–600 °C
	97.1	4.61	196.3	12.01	279.6, 311.7	15.98
RGO-1.0GD	25–130 °C		130–260 °C		260–600 °C
	83.8	5.85	192.5	10.72	387.9	18.78
RGO-2.0GD	25–130 °C		130–260 °C		260–600 °C
	97.4	6.18	198.3	9.51	400.0	22.27
RGO-3.0GD	25–130 °C		130–260 °C		260–600 °C
	70.0	6.56	192.5	8.92	410.7	29.42
RGO-4.0GD	25–130 °C		130–260 °C		260–600 °C
	67.9	6.98	192.5	8.47	418.2	37.53

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Fig. 4 shows an exponential increase of the weight losses in the third stage of degradation, i.e., the amounts of grafted PAMAM onto the surface of nanolayers with the generations of PAMAM (y = 15.393e^0.215x, where y is the weight losses (%) and x is the number of the generation, R² = 0.9872). As shown by the results, weight loss increases exponentially by generation progression which is the best proof of successful grafting of PAMAM dendrimers onto the surface.⁴¹

Fig. 4 The fitting curve of weight loss versus the generation.

XRD patterns of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD are shown in Fig. 5. Pristine graphite shows a basal reflection (002) peak at 2θ = 26.0° (d-spacing = 0.34 nm).⁴² After oxidation of graphite, the 002 reflection peak is shifted to a lower angle at 2θ = 9.5° (d-spacing = 0.93 nm), revealing that the d-spacing increases due to the intercalation of oxygen-containing moieties in between the basal plane of graphite.¹⁸ The XRD pattern of RGO-NH₂ shows that the 002 reflection peak has been shifted to a lower angle at 2θ = 8.4° (d-spacing = 1.05 nm) and the increase in d-spacing is not significant. Although the very small size of ethylenediamine molecules as surface modifiers could be the main reason for this phenomenon, stitching of graphene nanolayers may play an important role. Ethylenediamine contains two primary amine groups where each amine group can react predominantly with –COOH groups at the edge of nanosheets.²⁰ Reaction of both amine groups with different nanosheets leads to stitching of nanolayers that prevents their intercalation. Also, RGO-NH₂ shows a sharp peak at 2θ = 25.4° related to reduction of epoxy groups on the surface GO.³⁸ It can be concluded that in RGO-NH₂, ethylenediamine plays several roles such as reduction, surface modification and stitching of GO layers. A further modification process to obtain PAMAM-grafted nanosheets results in slightly increased d-spacing of 1.09 nm (2θ = 8.1°), 1.13 nm (2θ = 7.8°), 1.16 nm (2θ = 7.6°), and 1.21 nm (2θ = 7.3°) for RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD respectively. According to the results, no significant differences have occurred with respect to RGO-NH₂ and a slightly change in d-spacing shows that higher generation dendrimers make no difference in XRD patterns. It can be concluded that there is a resistance against intercalation of nanosheets and that may be the stitching of layers. Also, further modification decreases the intensity of the peak at 2θ = 25.4°; while it becomes broader due to the reduction effect of ethylenediamine. Because reduction starts from the edges of GO nanolayers and proceeds into the basal planes, during the reduction, parts of the basal planes near the edges become reduced and subsequently snap together due to π–π interactions. Consequently, the reducing agent, cannot penetrate further into the interior of the nanolayers and leads to the lower degree of reduction that results in a broadened XRD peak.^32,43,44

Fig. 5 XRD patterns of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD.

Raman spectroscopy as a non-destructive and highly-sensitive method to investigate the electronic structure is an essential tool for the characterization of carbon-based materials in which C=C bonds lead to high Raman intensities.⁴⁵ Thus, ordered and disordered crystal structure of carbon in G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD was studied by Raman spectroscopy (Fig. 6 and Table 2). The graphite spectrum shows three characteristic peaks around 1312 cm⁻¹ (disorder or D-mode), 1577 cm⁻¹ (tangential G-mode), and 2638 cm⁻¹ (2D or G′ band). The D-mode (breathing mode of κ-point phonons of A1g symmetry) is ascribed to defects in the graphene layers and the edge effect of graphene crystallites. Thus, a perfect graphite crystal should not exhibit the D band. However, for most commercial graphite products, high-temperature treatments during production introduce some defects and reduce crystallite sizes which results in increasing edge effects.⁴⁶ The G peak is attributed to the first order scattering of the E2g phonon of sp² carbon atoms.²⁵ The 2D (G′) band originates from the stacking order of nanosheets.⁴² The intensity ratio of D and G bands (I_D/I_G) as a measure of the quality of graphitization or defective disorders on the crystalline graphite is used to determine how modification disrupts the structure of graphite.⁴⁷ The I_D/I_G value is obtained as 0.32 for graphite which shows low disorder structure. Also, the 2D peak is located at 2639 cm⁻¹ with I_2D/I_G = 0.1. The D and G bands of GO appear at 1303 and 1582 cm⁻¹ respectively and the G band becomes broadened. The shift of the G band to a higher value and its broadening indicate the destruction of the sp² structure and the formation of defects in the graphene sheets due to oxidation and graphite’s amorphization.^18,48 The I_D/I_G value increases to 1.53 with predominant D band due to the reduction in size of the in-plane sp² domains originating from the oxidation process. Also, this may be ascribed to creation of new graphitic domains with smaller size.⁴⁹ The 2D peak shifts to a lower Raman shift of 2614 cm⁻¹ with I_2D/I_G = 0.06. The lower value of I_2D/I_G shows lower stacking of GO nanosheets with respect to G. The ratio of the D to G band intensities (I_D/I_G) is inversely proportional to the crystallite size (L_a) as depicted in eqn (1):⁸

where λ_laser is the laser excitation wavelength. Therefore, fewer defects result in a higher crystallite size and also a lower I_D/I_G value. It is obvious that after the oxidation process, L_a remarkably reduces to 59.6 nm. After treatment with ethylenediamine, some strange results are obtained; the G band shifts to higher values and the I_D/I_G value increases to 2.22 which disagree with the results from Lee et al.³⁸ Also, the 2D peak shifts to a higher Raman shift of 2750 cm⁻¹ with I_2D/I_G = 0.14. The G band shifts to a higher Raman shift and a higher I_D/I_G value shows more disordering in the surface of nanosheets originating from the reaction of ethylenediamine with some functional groups even at the edge or on the surface of graphene nanosheets. However, the 2D peak shift with a higher I_2D/I_G may be ascribed to the reduction effect of ethylenediamine. Although a lower I_D/I_G is expected due to reduction of oxygenated moieties, functionalization, which is proved by TGA and XRD, prevents the G band from shifting back or the I_D/I_Gdecreasing. Also, functionalization results in reduction of the crystallite size (L_a) to 41.0 nm. As shown in Fig. 6, new peaks appear after reaction of GO and ethylenediamine. Ethylenediamine skeletal bending appears at around 245 and 390 cm⁻¹ and the Raman shift at 637 cm⁻¹ is ascribed to NH₂ rocking. Secondary amines originating from the reaction of ethylenediamine with epoxide groups on the surface of nanosheets²⁰ show a Raman shift at 720 cm⁻¹. The peak at 844 cm⁻¹ has previously been assigned to the existence of ethylenediamine either in the pure state or monohydrate state⁵⁰ and represents a vibration arising from a coupling of skeletal stretching and NH₂ modes. Peripheral primary amines show a peak at around 950 cm⁻¹ and the peak at 1065 cm⁻¹ indicates that NH₂ is involved in the vibration.⁵⁰ After functionalization with PAMAM, the G band shows a slightly shift to 1587, 1588, and 1589 for RGO-1.0GD, RGO-2.0GD and RGO-3.0GD respectively. This shows that functionalization has occurred and is proved via an insignificant increase in I_D/I_G. However, as concluded from other results, it is performed non-ideally due to consumption of some ethylenediamine molecules in the reduction and stitching processes. Also, I_2D/I_G decreases after each step of treatment which shows functionalization is the most commonly occurring mechanism with respect to reduction and stitching processes. RGO-4.0GD shows no obvious change in peak locations, while I_D/I_G increases slightly due to functionalization and the 2D peak disappears.

Fig. 6 Raman spectra of G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD.

Table 2 Summarized Raman results relating to G, GO, RGO-NH₂, RGO-1.0GD, RGO-2.0GD, RGO-3.0GD, and RGO-4.0GD

Sample	D band (cm^−1)	G band (cm^−1)	2D band (cm^−1)	ID/IG	I2D/IG	La (nm)
G	1312	1577	2639	0.32	0.10	284.8
GO	1303	1582	2614	1.53	0.05	59.6
RGO-NH2	1309	1585	2750	2.22	0.14	41.0
RGO-1.0GD	1301	1587	2758	2.44	0.05	37.4
RGO-2.0GD	1297	1588	2769	2.51	0.05	36.3
RGO-3.0GD	1308	1589	2756	2.52	0.04	36.2
RGO-4.0GD	1305	1589	—	2.54	—	35.9

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To study the effect of modification steps on the morphology of samples, G, GO, RGO-NH₂, RGO-2.0GD, and RGO-4.0GD were analyzed by SEM. As shown in Fig. 7, graphite looks like thin “petal” flake with a typical lamella structure in which graphene layers are not distinguishable. In GO, the lamella structure is crumpled and wrinkled due to the oxidation process, while ultrathin and homogeneous graphene films exist. These nanosheets are folded and it is possible to distinguish the edges of individual sheets, including kinked and wrinkled areas. As seen in Fig. 7C, treatment of GO with ethylenediamine results in the disappearance of the folded structure and re-stacking of nanosheets. However, the edges of individual sheets are still distinguishable. Also, RGO-2.0GD and RGO-4.0GD show no significant change with respect to RGO-NH₂. For both of them, a lamella structure with distinguishable edges of nanosheets is illustrated. This shows a resisting phenomenon that prevents intercalation of nanosheets that has previously been reported as stitching³⁷ and formation of 3D network.⁵¹ These results are in good agreement with XRD and Raman results.

Fig. 7 SEM image of (A) G, (B) GO, (C) RGO-NH₂, (D) RGO-2.0GD, and (E) RGO-4.0GD.

The microstructure of different samples was also studied by means of TEM as depicted in Fig. 8. TEM study shows that graphite consists of transparent layers that stack together and no distinguishable graphene layer can be observed. After oxidation, the surface of GO becomes full of oxygen-containing groups. In TEM analysis, the opposing effects of π-conjugated domains and electrostatic repulsion of negative charges of oxygen-containing groups result in wrinkled and folded nanoplatelets, whereas GO nanosheets are distinguishable. According to the TEM images of RGO-NH₂ at different magnification (Fig. 8C and D), graphene nanosheets re-stack together (Fig. 8C) with distinguishable edge of layers (Fig. 8D). However, the lower transparency of RGO-NH₂ samples may be ascribed to modification of nanosheets by ethylenediamine. In RGO-4.0GD, although the edges of the nanosheets are distinguishable, the sample becomes less transparent with lots of defects on the surface. However, the higher-magnification TEM image of RGO-4.0GD (Fig. 8F) shows a number of spherical-like particles with sizes of 10–20 nm due to PAMAM dendrimers^52,53 covering the surface of the nanolayers. This is the main reason why the functionalization process has been performed⁴¹ while intercalation of nanosheets has not occurred significantly as described in the XRD results.

Fig. 8 TEM image of (A) G, (B) GO, (C & D) RGO-NH₂, and (E & F) RGO-4.0GD.

Conclusions

Natural graphite was oxidized via Hummers’ method to obtain GO. Then, ethylenediamine was grafted onto the GO surface. An iterative sequence of Michael addition and amidation reactions with ethylenediamine-treated GO nanosheets as primary core and methyl acrylate was used to synthesize PAMAM-grafted GO. It is found that ethylenediamine plays several roles comprising modification, reduction and stitching agent leading to an unexpected and non-ideal grafting of PAMAM onto GO nanolayers. According to the TGA results, the weight loss increased from 16.0 wt% for RGO-NH₂ to 18.8, 22.3, 29.4 and 37.5 wt% and T_d,max increased to 387.9, 400.0, 410.7 and 418.2 °C respectively for RGO-1.0GD, RGO-2.0GD, RGO-3.0GD and RGO-4.0GD. Weight loss increases showed functionalization takes place from non-stitching moieties. However, XRD results showed that stitching prevents intercalation of nanolayers. According to the Raman spectra, the I_D/I_G value raised with modification progression, whereas nanolayers were reduced. The main difference was observed via SEM and TEM images. Graphite showed a stacked structure of layers in which graphene layers were not distinguishable. In GO, the lamella structure was crumpled and wrinkled with homogeneous graphene films. Treatment of GO with ethylenediamine resulted in disappearance of the folded structure and re-stacking of nanosheets, while the edges of the sheets were still distinguishable. Also, no significant change occurred with respect to RGO-NH₂ with grafting PAMAM onto GO nanosheets and a lamella structure with distinguishable edges of nanosheets was illustrated. Conclusively, a non-ideal and unexpected modification occurred during a multi-step reaction.

Notes and references

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