materials Article
Composition, Structure and Morphology Evolution of Octadecylamine (ODA)–Reduced Graphene Oxide and Its Dispersion Stability under Different Reaction Conditions Tianjiao Bao 1 , Zhiyong Wang 1, *, Yan Zhao 2, *, Yan Wang 1 and Xiaosu Yi 1 1 2
*
Beijing Institute of Aeronautical Materials, Beijing 100095, China;
[email protected] (T.B.);
[email protected] (Y.W.);
[email protected] (X.Y.) Department of Materials Science and Engineering, Beihang University, Beijing 100191, China Correspondence:
[email protected] (Z.W.);
[email protected] (Y.Z.); Tel.: +86-134-0101-6439 (Y.Z.)
Received: 17 July 2018; Accepted: 10 September 2018; Published: 13 September 2018
Abstract: Octadecylamine (ODA) can solve the aggregation problem of graphene sheets in the chemical exfoliation method. However, no attempts have been made to investigate the evolution of ODA–reduced graphene oxide (ORGO) with reaction conditions and the modification mechanism, which is the core problem to realize the controllable production and practical application of graphene. In this study, we treated graphene oxide (GO) with ODA under different reaction conditions to prepare ORGO. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, thermogravimetric analysis (TGA), and UV–vis spectrophotometry were employed to analyze the composition, structure, morphology and characteristics of the as–prepared graphene sheets. The results showed that the reduction reaction could occur under mild conditions, but the edge grafting reaction could only be activated by a higher temperature. Moreover, the ORGO created at 80 ◦ C for 5 h and 120 ◦ C for 0.5 h exhibited the optimized properties, both excellent dispersing stability and high heat resisting property, since they had more edge grafting chains and a suitable reduction degree. Keywords: Octadecylamine (ODA); ODA–reduced graphene oxide; dispersity; reduction
1. Introduction Since its excellent properties were widely reported [1], graphene and its derivatives were closely concerned in nanocomposite, conductors, and film material area [2–6]. Of key importance in the realization of graphene’s full potential are the development of bulk production methods and improvement of its dispersibility in non-aqueous solvent. Currently, chemical exfoliation [7] is the only method to implement quantity production and application of graphene. In chemical exfoliation, graphene oxide (GO) is prepared first, and then it can be reduced to reduced graphene oxide (RGO) by chemical [8] or thermal methods [9]. However, the as-prepared RGO is usually thermodynamically unstable due to strong intersheet van der Waals (vdWs) attractive forces and is hard to be exfoliated and stably dispersed in solvent. Much work [10,11] on exfoliation and dispersion of RGO in nonaqueous solvent was developed, since it may significantly facilitate the practical use of graphene, such as preparing nanocomposites, film materials and additives in lubricant. Adding stabilizers [12,13] is one method to improve the dispersion of graphene in solvent, but the presence of these stabilizers is not desirable for most applications. Another approach is to modify RGO by grafting, which usually has two steps: Grafting after the reduction or reduction after the grafting. For example, Materials 2018, 11, 1710; doi:10.3390/ma11091710
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Ruoff et al. [14] functionalized GO with isocyanate to prevent aggregation in the reduction process with N,N-dimethylhydrazine. Nonpolar chemicals with long alkyl chains, such as octadecylamine (ODA), have been widely used to functionalize GO to improve its dispersion in nonaqueous solvent [15–20]. In previous works, ODA was mainly used as a surface modifier to improve the hydrophobicity of GO and its dispersion in non-polar polymers. For instance, Petridis et al. [16] reduced graphite oxide (GO) into graphite by either NaBH4 or hydroquinone, and then modified its surface with neutral, primary aliphatic amines and amino acids. Feng et al. [18] reported to reduce GO with hydrazine and then refluxed the reduced GO in a DMF solution of ODA at 90 ◦ C for 48 h to improve its lipophilicity. Wang et al. [19] synthesized organophilic graphene nanosheets by modifying GO with ODA for 20 h, and then chemical reducing modified product with hydroquinone for another 22 h. Recently, Wenjuan Li et al. [21] demonstrated an approach to functionalize and in situ reduce GO with ODA without another reducing agent, which would simplify the process of producing solvent soluble RGO; this brought the promising future for the practical use of graphene. For practical application, as the additive for nanocomposite for example, the ideal structure of modified GO was both with high degree of reduction (for excellent mechanical properties, thermal properties and electrical properties) and excellent dispersing performance in non-aqueous solvent (for good dispersion in composite). It evoked our interest to know how to realize the ideal morphology, structure and properties of ODA modified RGO by controlling reaction conditions. As a consequence, more significant work should be completed to explore the mechanism of modification and trace the evolution of ODA–reduced graphene with reaction conditions, so that the structure of ORGO could be controllable and its properties could be designable. It was known to all that graphene’s final properties were significantly affected by various factors, such as defect, dimension, number of layers, grafting degree, reduction degree, etc. For example, it was easy to crease of the graphene with great dimension, which had a higher friction factor than the graphene with smaller dimensions. The graphitic structure’s higher degree of recovery could lead to better thermal property; however, the introduction of structure’s defects could exert a negative influence on the thermal property. In some circumstances, factors were closely connected and conflicted with each other. Therefore, we should thoroughly investigate the evolution of graphene with condition as well as the influencing and constrained relationships among various factors, understanding modification mechanism, so as to realize the design and control of materials’ properties. Furthermore, the treatment of GO with ODA in previous studies was usually under drastic conditions, i.e., conditions with high reaction temperature and long reaction time. Nevertheless, we expected to use a simple and high–efficiency process to produce graphene. As a result, we studied the structure, composition and morphological evolution of ODA–reduced GO (ORGO) in various conditions in our research. The dispersing performance and thermal properties were evaluated respectively to further comprehend and verify the evolution. Based on the study, we were able to obtain the desired RGO and realize its practical application by controlling conditions. In our study, we determined that a certain degree of reduction can occur at a low temperature (35 ◦ C) in a short time (0.5 h), which can lower the production cost. Significantly, we identified the conditions to produce RGO with both high reductive degree and long–term dispersion stability. This result will facilitate the preparation of graphene–polymer composites and/or the development of graphene–based hybrid materials. 2. Materials and Methods 2.1. Raw Materials GO was provided by Beijing institute of aeronautical materials, which was prepared by Hummer’s method. Octadecylamine (ODA), hydrazine hydrate (HHA, 85.0%), alcohol, tetrahydrofuran (THF), were analytical reagent grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).
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2.2. Modification of GO GO was prepared from graphite using the modified Hummer’s method [22], which was characterized by AFM (Bruker, Billerica, MA, USA), TEM and XRD in Figure S1 of Supplementary Materials. To be treated by modifier, 0.1 g of GO was added into 200 mL of absolute ethyl alcohol, and the GO/alcohol colloidal suspension was subjected to ultrasonication for 30 min. Then, 0.5 g of ODA was added into the solution with magnetic stirring and was refluxed under different reaction conditions. The prepared reduced graphene oxide (RGO) was washed 10 times by absolute ethyl alcohol together with suction filtration. At last, the filtered material was dried under vacuum (100 ◦ C, 3 h). The ORGO film was obtained by filtration using a nylon membrane (0.04 µm, Whatman, Kent, England). The GO film was also prepared as the above method without adding ODA. The investigated reaction conditions were 35 ◦ C, 0.5 h; 35 ◦ C, 5 h; 35 ◦ C, 24 h; 50 ◦ C, 0.5 h; 50 ◦ C, 5 h; 80 ◦ C, 0.5 h; 80 ◦ C, 5 h; 120 ◦ C, 0.5 h; and 120 ◦ C, 5 h, respectively. To analyze the structure and morphology of GO and RGOs, the samples of different conditions. were labelled. The GO was labeled as a, and ODA modified RGO with 35 ◦ C, 0.5 h; 35 ◦ C, 5 h; 35 ◦ C, 24 h; 50 ◦ C, 0.5 h; 50 ◦ C, 5 h; 80 ◦ C, 0.5 h; 80 ◦ C, 5 h; 120 ◦ C, 0.5 h; 120 ◦ C, 5 h were named as b, c, d, e, f, g, h, and i, successively. All the characterizations of GO and RGOs in the result section were according to this order. The sample prepared at 35 ◦ C for 24 h was only characterized by XPS to explain a certain conclusion, which was named as c-1. As a comparison, hydrazine hydrate (HHA) was used to replace ODA in the reduction reaction, and HHA reduced graphene oxide (HRGO) was prepared at 80 ◦ C for 5 h, named as j. 2.3. Material Characterizations General characterizations of the graphene oxide and reduced graphene oxide product were carried out by means of thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy (Renishaw in Via, Gloucestershire, UK). The original material GO was characterized by transmission electron microscope (TEM) using Tecnai G2 F20, FEI corporation (Hillsboro, OR, USA), and X-ray diffraction (XRD) using D8 advance (Bruker corporation, Billerica, MA, USA). The as–prepared reduced graphene oxide was characterized by atomic force microscopy (AFM) to show the particle size of the microscopic particle. The dispersion of RGO in tetrahydrofuran (THF) was observed after natural precipitation by digital pictures and UV–vis absorption spectroscopy (Shimadzu, Kyoto, Japan). XPS measurements were taken using a Thermo Escalab 250XI (Hillsboro, OR, USA) under 1 × 10−10 mbarwith monochromatic Al KR and an X-ray source with a power of 150 W. FTIR spectra were recorded with a Nicolet iS10 FTIR spectrometer (Hillsboro, OR, USA) using KBr pellets with a sample concentration of ~0.1 wt%. TGA was carried out using an SDTQ600 thermobalance (TA Instruments, New Castle, PA, USA) in air at a heating rate of 10 ◦ C·min−1 . UV–vis absorption spectra were recorded on a UV–2550 spectrophotometer (Shimadzu, Kyoto, Japan). AFM images were obtained using a Multimode8 instrument (Bruker Corporation) in a scan assist mode. Samples for the AFM imaging were prepared by drop–casting the dispersions onto freshly cleaved mica substrates (grade V–1, Electron Microscopy Sciences, Hatfield, PA, USA), which were then allowed to dry in air. Raman spectra were recorded using a Renishaw invia with a laser wavelength of 632.8 nm. The wettability of GO and rGO was tested by contact angle meter using DSA25 of Kruss Corporation (Wichita, KS, USA). Contact angle was measured right after dropping a 3 µL water droplet on each film. 3. Results 3.1. Composition and Chemical Structure of ORGOs The chemical changes occurring upon the treatment of GO with ODA can be observed by FTIR spectroscopy as both GO and its ODA–treated derivatives display characteristic IR spectra. The FTIR spectrum of GO (Figure 1a) was very typical [16,23,24]. The characteristic features in the FTIR spectrum of GO were the adsorption bands that correspond to the C=O bond of the carbonyl or carboxyl groups at
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1727Materials cm−1 , the deformation vibration at 1373 cm−1 , the C–OH stretching at 1226 cm−1 , and4 the 2018,O–H 11, x FOR PEER REVIEW of 15C–O − 1 stretching at 1050 cm . However, the FTIR spectra of ORGO were markedly different from that of GO. at 1727 cm−1, the O–H deformation vibration 1373 the C–OH at −1 ) −1−1, ,1226 The carboxyl signals ofgroups oxygen–containing functional groups (1727 cm−1 , at 1373 cmcm cm−1 , stretching and 1050 cm −1 −1 1226 cm , and the C–O stretching at 1050 cm . However, the FTIR spectra of ORGO were markedly almost disappeared, which suggested the graphene oxide was reduced by ODA. Instead, the characteristic different from that of GO. The signals of oxygen–containing functional groups (1727 cm−1, 1373 cm−1, peaks of long–chain alkyls and amines appeared. As a reference, The FTIR spectrum of ODA was shown 1226 cm−1, and 1050 cm−1) almost disappeared, which suggested the graphene oxide was reduced by in Figure S2 (in Supplementary Materials). GO treated under mild conditions (Figure 1b–f) had similar ODA. Instead, the characteristic peaks of long–chain alkyls and amines appeared. As a reference, The −1 , 2850–2919 cm−1 , 1643 cm−1 , 1467 cm−1 , 1100 cm−1 characteristic peaks, bands at 3320–3280 FTIR spectrum of i.e., ODA was shown in Figurecm S2 (in Supplementary Materials). GO treated under mild 1 and conditions 720 cm−1 , respectively. at 3280–3320 cm− associated with thecm stretching of N–H −1, 2850–2919 (Figure 1b–f) The had single similarpeak characteristic peaks, i.e.,isbands at 3320–3280 −1, 1100 cm−1 and −1, respectively. of secondary The peak bands was totally from multi-peak cm−1, 1643 amine. cm−1, 1467 cmcharacterized 720 at cmthese The singledifferent peak at 3280–3320 cm−1 is in spectrum of ODA demonstrated reaction between graphene oxide and ODA. Thebands new bands associated withwhich the stretching of N–Hthe of secondary amine. The characterized peak at these was at − 1 − 1 totally cm different multi-peak in spectrum ODA which demonstrated the 2850–2919 and from 1467 cm were the stretchingofand deformation vibrations of thereaction C–H inbetween methyl and −1 and 1467 cm−1 were the stretching and graphene oxide respectively. and ODA. The new bands at 2850–2919 methylene groups, The peak at 720 cm−1 wascm a characteristic peak of the linear connection deformation vibrations of the C–H methyl andpeak methylene The peak atwith 720 the in methyl groups. Furthermore, thereinwas a new at 1100groups, cm−1 , respectively. which was associated −1 was a characteristic peak of the linear connection in methyl groups. Furthermore, there was a cm vibration of the C–N groups. This result was also confirmed by the XPS analysis and demonstrated the new peak at 1100 cm−1, which was associated with the vibration of the C–N groups. This result was successful chemical reduction and surface grafting by ODA. The modification of GO by ODA was also also confirmed by the XPS analysis and demonstrated the successful chemical reduction and surface reflected in Contact angle result (in Figure 2) indirectly. As we can see, after ODA functionalization, grafting by ODA. The modification of GO by ODA was also reflected in Contact angle result (in the removal of oxygenic group and the attaching of aliphatic chains made the ODA-GO film hydrophobic. Figure 2) indirectly. As we can see, after ODA functionalization, the removal of oxygenic group and Thus, contact of angle increased 100the (asODA-GO reportedfilm in previous studyThus, [20], the the contact contactangle angle of thethe attaching aliphatic chainsover made hydrophobic. GO increased was lower than Considering that the reduction ofofhydrazine hydrate over 10060). (as reported in previous study [20], themechanism contact angle GO was lower thanreduced 60). graphene oxide in a former study [25] and the characteristics of primary amine groups, we Considering that the reduction mechanism of hydrazine hydrate reduced graphene oxide in aspeculated former that study the reduction occurred throughoftwo probable routes (shown in Scheme 1).the In reduction general, the reduction [25] and the characteristics primary amine groups, we speculated that occurred through probablefrom routes (shown in Scheme 1). between In general, the reduction reaction wasthe originated reaction wastwo originated nucleophilic addition amine and epoxy in both two routes. nucleophilic addition between amine the andamine epoxy in both the twoasroutes. In a typical nucleophilic In a from typical nucleophilic addition reaction, group acted a nucleophile and attacked the addition reaction, the amine group acted as a nucleophile and attacked the carbon atom in epoxy carbon atom in epoxy groups, grafting the long hydrocarbon chain of the ODA onto GO sheets. However, groups, grafting the long hydrocarbon chain of the ODA onto GO sheets. However, the deoxidation the deoxidation mechanism was different in route 1 and route 2. Route 1 was de–epoxidation process, mechanism was different route 1 and route 2. Routeby 1 was de–epoxidation process, in whichposition the in which the long–chain wasin exfoliation from graphene reacting with hydroxyl at adjacent long–chain was exfoliation from graphene by reacting with hydroxyl at adjacent position and the and the deoxygenated product was then produced. On the other hand, when the epoxide group was deoxygenated product was then produced. On the other hand, when the epoxide group was adjacent adjacent to a hydroxyl group, the mechanism of route 2 might take place. In route 2, the epoxide ring to a hydroxyl group, the mechanism of route 2 might take place. In route 2, the epoxide ring was was opened opened as aswell wellfor forthe thefirst firststep, step,while whileininthe thenext next step, two adjacent hydroxyls reacted and H2 O was step, two adjacent hydroxyls reacted and H2O was eliminated instead of the long–chain. eliminated instead of the long–chain.
◦ C, Figure 1. FTIR spectra GOand andORGOs ORGOs (a: at at 35 35 °C,◦0.5 ORGO at 35 °C, 5 h; d: 5 h; Figure 1. FTIR spectra ofofGO (a: GO; GO;b:b:ORGO ORGO C, h; 0.5c:h; c: ORGO at 35 ◦ ◦ ◦ ◦ ORGO at 50 °C, 0.5 h; e: ORGO at 50 °C, 5 h; f: ORGO at 80 °C, 0.5 h; g: ORGO at 80 °C, 5 h; h: ORGO d: ORGO at 50 C, 0.5 h; e: ORGO at 50 C, 5 h; f: ORGO at 80 C, 0.5 h; g: ORGO at 80 C, 5 h; at 120 at °C,120 0.5 ◦h;C,i:0.5 ORGO 120 °C,at5120 h). ◦ C, 5 h). h: ORGO h; i: at ORGO
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Figure ((b) ORGO at 35 0.5 h; (g) ORGO at 80 h). Figure 2. The water contact angle of typical ORGO ((b) ORGO ORGO at at 35 35 ◦°C, °C, 0.5 h; h; (g) (g) ORGO ORGO at at80 80◦°C, °C, h). Figure 2. 2. The Thewater water contact contact angle angle of of typical typical ORGO ORGO ((b) C, 0.5 C, 555 h).
Scheme Scheme 1. 1. Deoxygenation Deoxygenation mechanism mechanism of of GO GO reduced reduced by by ODA. ODA.
The IRspectra spectraof the RGO produced a reaction temperature h or 80 above 80 ◦ C The RGO produced at temperature of for or °C The IR IR spectra ofofthe the RGO produced at aaatreaction reaction temperature of 80 80of for8055 h hfor or 5above above 80 °C (Figure (Figure −1 (Figure 1g–i) had completely characteristic peaks. An absorption at 1000–1700 cm−1 1g–i) completely different characteristic peaks. absorption valley 1000–1700 cm 1g–i) had had completely differentdifferent characteristic peaks. An An absorption valley at at valley 1000–1700 cm−1 appeared appeared appeared which was attributed to the association of the hydrogen bond the amide group. Obviously, which attributed to of bond the amide group. Obviously, the which was was attributed to the the association association of the the hydrogen hydrogen bond in in the in amide group. Obviously, the the amide was formed by the reaction of –COOH at the edge of GO with the –NH of ODA (named 2 2 amide was formed by the reaction of –COOH at the edge of GO with the –NH of ODA (named edge amide was formed by the reaction of –COOH at the edge of GO with the –NH2 of ODA (named edge edge grafting). This indicated that the reaction thecarboxyl edge carboxyl couldbe be activated grafting). This that reaction of edge group could activated under grafting). This indicated indicated that the the reaction of the the of edge carboxyl group group could only only beonly activated under under certain conditions. That was why GO needed to be activated by sulfoxide chloride when certain conditions. That was why GO needed to be activated by sulfoxide chloride when ODA was certain conditions. That was why GO needed to be activated by sulfoxide chloride when ODAODA was was used as a surfactant [25,26]. To give an explanation, the mechanism of edge grafting reaction used as a surfactant [25,26]. To give an explanation, the mechanism of edge grafting reaction used as a surfactant [25,26]. To give an explanation, the mechanism of edge grafting reaction was was compared Route 1 of Scheme 1 to11 Scheme 2, we that that the two to that of reduction reaction. From Route 11 of Scheme to 2, we the compared to to that thatof ofreduction reductionreaction. reaction.From From Route of Scheme to Scheme Scheme 2, figured we figured figured that the reactions all went through two steps, in which the first step was the nucleophilic addition reaction of two reactions all went through two steps, in which the first step was the nucleophilic addition two reactions all went through two steps, in which the first step was the nucleophilic addition oxygen containing groups and formed the same chemical bond. The only difference was that the epoxy reaction reaction of of oxygen oxygen containing containing groups groups and and formed formed the the same same chemical chemical bond. bond. The The only only difference difference was was ring was opened in the reduction reaction, while the carbonyl group was broken in the edge grafting that the epoxy ring was opened in the reduction reaction, while the carbonyl group was broken that the epoxy ring was opened in the reduction reaction, while the carbonyl group was broken in in reaction. to the carbonyl group, the carbonyl carbon–oxygen in epoxy has higher bond energy, the grafting Compared to group, the bond epoxy has the edge edge Compared grafting reaction. reaction. Compared to the the carbonyl group, bond the carbon–oxygen carbon–oxygen bond in in epoxy has so the activation energy ofactivation epoxy nucleophilic addition would beaddition lower. According to the kinetics of higher bond so energy nucleophilic would According higher bond energy, energy, so the the activation energy of of epoxy epoxy nucleophilic addition would be be lower. lower. According chemical reaction, a higher temperature needed towas activate the molecules overcome to of reaction, aa higher temperature needed to activate reacting molecules to to the the kinetics kinetics of chemical chemical reaction, higherwas temperature was needed toreacting activate the the reactingto molecules to the higher energy barrier (activation energy). Therefore, the edge grafting reaction needed a higher overcome the higher energy barrier (activation energy). Therefore, the edge grafting reaction needed overcome the higher energy barrier (activation energy). Therefore, the edge grafting reaction needed aa temperature to be activated. higher to higher temperature temperature to be be activated. activated.
Scheme Scheme 2. 2. Reaction Reaction mechanism mechanism of of ODA ODA grafting grafting onto onto GO. GO.
An An XPS XPS analysis analysis was was performed performed to to further further investigate investigate the the reaction reaction between between GO GO and and ODA, ODA, and and the results were consistent with the IR spectra, which demonstrated the successful chemical reduction the results were consistent with the IR spectra, which demonstrated the successful chemical reduction
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An XPS analysis was performed to further investigate the reaction between GO and ODA, and the Materials 2018, 11, x FOR PEER REVIEW 6 of 15 results were consistent with the IR spectra, which demonstrated the successful chemical reduction and surface modification of RGO.ofFigure was the3 deconvolution of the peaks’ curves of XPS spectrum and surface modification RGO.3 Figure was the deconvolution of the peaks’ curves of XPSof GOspectrum and ORGOs. brief, the C 1s GOspectrum had three that corresponded of GOInand ORGOs. In XPS brief,spectrum the C 1s of XPS ofcomponents GO had three components thatto thecorresponded carbon atomstointhe thecarbon different functional groups, i.e., the nonoxygenated ring C (284.5 eV), the atoms in the different functional groups, i.e., the nonoxygenated ring CC (284.5 eV), C bonds in the C–O C–N eV), andbonds the C in theeV) C=O bondsAlthough (288 eV) [27,28]. in the C–O or the C–N (286 or eV), andbonds the C(286 in the C=O (288 [27,28]. the C 1s 1sORGO XPS spectrum of the ORGO also exhibited these same oxygen functionalities, XPSAlthough spectrumthe ofCthe also exhibited these same oxygen functionalities, the peak intensities the were peak intensities were much less than those in GO, and the increase in the C–C/C=C signal was much less than those in GO, and the increase the C–C/C=C signal was evidence of the graphitic evidence of the graphitic carbon network restoration. This result confirmed the reduction reaction carbon network restoration. This result confirmed the reduction reaction mentioned in the IR spectra mentioned in the IR spectra analysis. Furthermore, to investigate reductive degree of RGO under analysis. Furthermore, to investigate the reductive degree of RGOthe under different reaction conditions, reaction conditions, ratio of different wasofobtained bypeak usingareas the ratio thedifferent ratio of different carbonate the bonds was obtainedcarbonate by using bonds the ratio the bond in the of the bond peak areas in the XPS spectra (Table 1), and data of RGO treated by hydrazine hydrate XPS spectra (Table 1), and data of RGO treated by hydrazine hydrate (HRGO) and GO were used as (HRGO) and GO were used as comparisons (the deconvolution of the peaks’ curve of HRGO was comparisons (the deconvolution of the peaks’ curve of HRGO was shown in Figure S6 of Supplementary shown in Figure S6 of Supplementary Materials). The results showed that the RGO created at 35 °C Materials). The results showed that the RGO created at 35 ◦ C for 0.5 h already had a high ratio of C–C for 0.5 h already had a high ratio of C–C bonds, i.e., 70% from 39% of GO, and the reductive degree bonds, i.e., 70% from 39% of GO, and the reductive degree (refer to the percentage of C=C/C–C) slightly (refer to the percentage of C=C/C–C) slightly increased with higher temperatures and longer times increased with higher temperatures and longer times (from 70% to 90%). This result was confirmed (from 70% to 90%). This result was confirmed by TGA analysis and indicated that the reduction of by GO TGAbyanalysis and indicated that the reduction of GO by ODA could occur under mild conditions. ODA could occur under mild conditions. Comprehensive performances of ORGO created at Comprehensive performances ORGO at 35 ◦ C for 0.5Materials). h were shown in Figures S3 and 35 °C for 0.5 h were shown in of Figures S3 created and S4 (Supplementary It proved that ORGO canS4 (Supplementary Materials). It proved that ORGO can bemeaningful produced with ODA without heating, which was be produced with ODA without heating, which was for quantity production. Moreover, ◦ C for 24 h (its meaningful for quantity production. Moreover, the reductive degree of RGO at 35 the reductive degree of RGO at 35 °C for 24 h (its deconvolution of the peaks’ curve was added in deconvolution of the peaks’ curve Supplementary Materials) was that of thealso RGO Supplementary Materials) was was closeadded to thatinof the RGO created at 80 °C forclose 5 h, to which was created at 80 ◦ Ctoforthat 5 h,ofwhich was also 1j). comparable to that of the HRGO 1j). This revealed that the time comparable HRGO (Table This revealed that time(Table and temperature were equivalent andfor temperature wereofequivalent the has reduction of GO, and ODA has the same reduction the reduction GO, and for ODA the same reduction efficiency as HHA under efficiency the sameas HHA under the same conditions. Theequivalent time–temperature equivalent phenomenon also conditions. The time–temperature phenomenon was also discoveredwas from thediscovered data of ◦ C for1h). ORGO created at 80 °C for 5 at h (Table 1g)5and ORGO at 120created °C for 0.5 h (Table from the data of ORGO created 80 ◦ C for h (Table 1g)created and ORGO at 120 0.5 h (Table 1h).
Figure 3. The deconvolution of the peaks in XPS C 1sCspectroscopy of GO ((a) GO; ORGO Figure 3. The deconvolution of the peaks in XPS 1s spectroscopy of and GO RGOs and RGOs ((a) (b) GO; (b) ◦ C, 0.5 at 35 at ORGO 35 ◦ C, 5ath;35(d) atORGO 50 ◦ C, at 0.550h;°C, (e) 0.5 ORGO 50 ◦ C,at5 50 h; (f) ORGO at h; 35 (c) °C,ORGO 0.5 h; (c) °C,ORGO 5 h; (d) h; (e)atORGO °C,ORGO 5 h; (f) at ◦ C, 5 h; ◦ C, at at 80 0.5 h;at(g) at (h) 80 °C, 5 h; (h) ORGO 0.5 h; (i)atORGO at5120 80 ◦ORGO C, 0.5 h; (g)°C, ORGO 80ORGO ORGO at 120 0.5120 h; °C, (i) ORGO 120 ◦ C, h).°C, 5 h).
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Table 1. The aspect ratio of different C in the XPS C 1s spectra. Table 1. The aspect ratio of different C in the XPS C 1s spectra. No. Notes C–O/C–N C=O C=C/C–C a GO 50 10C=O 39 No. Notes C–O/C–N C=C/C–C b ORGO (35 °C, 0.5 h) 24 6 70 a GO 50 10 39 ◦ C, 5 ORGO h)h) 2024 4 6 76 70 bc ORGO(35 (35°C, 0.5 ◦ C, cc-1 ORGO h) ORGO (35(35 °C, 245 h) 1820 4 4 78 76 ◦ C, 24 h) c-1d ORGO (35 18 4 ORGO (50 °C, 0.5 h) 19 6 75 78 d ORGO (50 ◦ C, 0.5 h) 19 6 75 e ORGO (50 °C, 5 h) 21 3 76 e ORGO (50 ◦ C, 5 h) 21 3 76 ◦ C,0.5 ORGO (80 1919 6 6 75 75 ff ORGO (80°C, 0.5h) h) ◦ C,55h) ORGO (80 1717 4 4 79 79 gg ORGO (80°C, h) ◦ C, 0.5 h) hh ORGO (120°C, ORGO (120 0.5 h) 1616 4 4 80 80 ◦ C, 5 h) ii ORGO (120 8 2 ORGO (120 °C, 5 h) 8 2 90 90 j HRGO (80 ◦ C, 5 h) 20 5 75 j HRGO (80 °C, 5 h) 20 5 75
Signals at 399.5 eV and 401.4 ev corresponding to N–C and N–H appeared in N1s XPS spectrum of ofof the peaks’ curves is of ORGO, ORGO, which which were werenot notobserved observedininthe thespectrum spectrumofofGO. GO.The Thedeconvolution deconvolution the peaks’ curves shown in Figure 4. The N–C signal represented the grafting effect of ODA onto GO, that is, the more is shown in Figure 4. The N–C signal represented the grafting effect of ODA onto GO, that is, the grafting chains,chains, the more intensive the N–C The quantitative data data in Table 2 showed that more grafting the more intensive the signal. N–C signal. The quantitative in Table 2 showed ◦ when the temperature waswas higher than 80 80C,°C, there was signal. that when the temperature higher than there wasananobvious obviousincrease increaseof of the the N–C signal. These observations were consistent with the IR spectrum data (Figure 1.) and indicated that edge IR spectrum data (Figure 1.) ◦ grafting occurs occurs at at80 80°C.C.The The N–C signal increased a longer (b to c-1 in 2), Table 2), N–C signal alsoalso increased withwith a longer time time (b to c-1 in Table which which withdispersion the dispersion results. deconvolution of the peaks’ curveofofc-1 c-1was was added added in agreedagreed with the results. (The(The deconvolution of the peaks’ curve Supplementary S5).S5). As we previously that thethat reduction reaction occurred SupplementaryMaterials, Materials,Figure Figure As speculated we speculated previously the reduction reaction in two steps: The steps: ring-opening reaction occurred firstly, and the aliphatic attached onto the RGO occurred in two The ring-opening reaction occurred firstly, and chains the aliphatic chains attached in thisthe step. Then the oxygenic was reduced bywas eliminating or water. amine To obtain excellent onto RGO in this step. Thengroup the oxygenic group reduced amine by eliminating or water. To dispersing performance, we expected to control the reaction condition, for instance by lowering the obtain excellent dispersing performance, we expected to control the reaction condition, for instance temperature theor reaction time to it at the first the density by lowering or theshortening temperature shortening thekeep reaction time to step, keep so it that at the first step, of soaliphatic that the chains on ORGO would be higher. However, the N–C signal was not stronger at low temperature density of aliphatic chains on ORGO would be higher. However, the N–C signal was not stronger or at short time, and the degree wasreduction high at mild condition. It pointed out that the low temperature orreduction short time, and the degree was high at mild condition. It deoxidation pointed out step occurred instantly as occurred epoxide ring was opened evenring at low temperature, soatthat was impossible that the deoxidation step instantly as epoxide was opened even lowittemperature, so to control the reduction reaction to suspend at the first step. that it was impossible to control the reduction reaction to suspend at the first step.
Figure 4. The deconvolution of the peaks in XPS N 1s spectroscopy of RGO (b: ORGO at 35 ◦°C, C, 0.5 h; ORGOatat3535◦ C, °C,5 5h;h; ORGO at ◦50 e: ORGO 505°C, h; f: ORGO h; g: c: ORGO d: d: ORGO at 50 C,°C, 0.5 0.5 h; e:h;ORGO at 50at◦ C, h; f:5ORGO at 80 ◦at C, 80 0.5 °C, h; g:0.5 ORGO ◦ ◦ ◦ ORGO at 80 °C, 5 h; h: ORGO at 120 °C, 0.5 h; i: ORGO at 120 °C, 5 h), the red line was deconvolution at 80 C, 5 h; h: ORGO at 120 C, 0.5 h; i: ORGO at 120 C, 5 h), the red line was deconvolution peak of peak at of 399.5 N–C eV at 399.5 eV blue and the was N–H ev. at 401.4 ev. N–C and the lineblue wasline N–H at 401.4
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Table 2. The aspect ratio of different N in the XPS N 1s spectra. Table 2. The aspect ratio of different N in the XPS N 1s spectra. No. b c c-1 d e f g h i
No. b c c-1 d e f g h i
Notes Notes ORGO (35 °C, 0.5 h) ◦ C, 0.5 ORGO °C,h)5 h) ORGO (35 (35 ◦ C, 5 h) ORGO (35(35 ORGO °C, 24 h) ◦ C, 24 h) ORGO (35(50 ORGO °C, 0.5 h) ORGO (50 ◦ C, 0.5 h) ORGO (50 °C, 5 h) ORGO (50 ◦ C, 5 h) ◦ C,°C, ORGO ORGO (80(80 0.5 0.5 h) h) ◦ C,°C, ORGO ORGO (80(80 5 h)5 h) ◦ C, 0.5 h) ORGO (120 ORGO (120 °C, 0.5 h) ORGO (120 ◦ C, 5 h) ORGO (120 °C, 5 h)
N–C N–C 36.1 42.7 36.1 42.7 49.5 49.5 20.4 20.4 31.9 31.9 40.7 40.7 55.7 55.7 7272 8484
N–H 63.9 57.3 50.5 79.6 68.1 59.3 44.3 28 16
N–H 63.9 57.3 50.5 79.6 68.1 59.3 44.3 28 16
3.2. Morphology Morphology Evolution Evolution of of ORGOs ORGOs with with Different Different Conditions 3.2. Conditions To investigate sheet dimensions dimensions of of ORGO ORGO under under different different reaction reaction To investigate the the degree degree of of exfoliation exfoliation and and sheet conditions in THF, AFM imaging of the dispersions deposited onto mica substrates was carried conditions in THF, AFM imaging of the dispersions deposited onto mica substrates was carried out. out. Representative were in shown 5. images The AFM images theirregularly presence Representative results results were shown Figurein5. Figure The AFM revealed the revealed presence of of irregularly shaped sheets with a uniform thickness ranged from onenanometers, nanometer which to six shaped sheets with a uniform thickness that ranged from that one nanometer to six nanometers, indicated that ORGOinto canone be exfoliated into one layer up to sheets indicated thatwhich ORGO can be exfoliated layer up to six–layer sheets in six–layer THF. This was in THF. This was important most ofproperties the attractive propertiesand of graphene and graphene–based important because most of because the attractive of graphene graphene–based sheets were sheets were mainly with associated their existence as individually separated More significantly, mainly associated their with existence as individually separated entities.entities. More significantly, after after modification, the particle of ORGO was greatly reduced compared GO to (refer to theimage TEM modification, the particle size ofsize ORGO was greatly reduced compared to GO to (refer the TEM image of S1 Figure S1 in Supplementary Materials). This phenomenon could be interpreted by the of Figure in Supplementary Materials). This phenomenon could be interpreted by the following following speculation. The recovery reaction of new double bonds was accompanied with a large speculation. The recovery reaction of new double bonds was accompanied with a large number of number ofbonds chemical bonds breaking, which had aoff” “cuteffect off” effect on the sheets and reducedthe the sheet chemical breaking, which had a “cut on the GOGO sheets and reduced dimensions. The probable Scheme 3. 3. When elimination of probable mechanism mechanismof of“cut “cutoff” off”effect effectwas wasshown showninin Scheme When elimination H2H O2from two hydroxy happened at two adjacent carbon, the the bond between themthem would break and of O from two hydroxy happened at two adjacent carbon, bond between would break formform a defect. ThenThen amounts of defects linked together, and the was was cut into slices. It was and a defect. amounts of defects linked together, andsheet the sheet cut little into little slices. It obvious that the reduced size would also be beneficial for the stability of suspension. was obvious that the reduced size would also be beneficial for the stability of suspension. spectroscopy is a powerful, nondestructive nondestructive tool tool that that can be used to characterize Raman spectroscopy carbonaceous materials, particularly distinguishing ordered and disordered crystal structures of carbonaceous materials, particularly distinguishing ordered and disordered 6. carbon. Here, Raman measurements measurementswere wereperformed, performed,and andthe thetypical typicalspectra spectraare arepresented presentedininFigure Figure −1 Each spectrum showed three main peaks [29]. 6. Each spectrum showed three main peaks [29].The TheGGband bandatatapproximately approximately1580 1580cm cm−1 originated 2 carbon atoms and is a doubly degenerate phonon mode 2 carbon from the the in–plane in–planevibration vibrationofofthe thespsp atoms and is a doubly degenerate phonon mode (E2g −1) originated (E2g symmetry) the Brillouin zone center. The 2D (at band (at approximately 2700 cm−1 ) originated symmetry) at theatBrillouin zone center. The 2D band approximately 2700 cm from a −1 was from a two–phonon resonance the Dat peak at approximately 1355 cm−a1 two–phonon double double resonance RamanRaman process,process, and theand D peak approximately 1355 cm was a breathing thesymmetry A1g symmetry that involved the phonons nearKthe K zone boundary. breathing mode mode of the of A1g that involved the phonons near the zone boundary. The The formation of defects was monitored byratio the ratio the intensities DG and G bands formation of defects was monitored by the of theofintensities of theofDthe and bands (ID/IG)(Iin the D /I G) in the Raman spectra, as shown in Table 3. The results showed that under mild reaction conditions Raman spectra, as shown in Table 3. The results showed that under mild reaction conditions (Figure (Figure 6b–f), the defects in RGO decreased the restoration of the graphiticcarbon carbonnetwork networkof of the 6b–f), the defects in RGO decreased sincesince the restoration of the graphitic The order order degree degree of sample at intense condition (Figure 6g,h) was destroyed destroyed by reduction reaction. The long–chain grafting onto graphene of asespecially especially high. high. However, However, since since the reductive of which which IIDD/I /IGGas ◦ C for 5 h, its ID /IGGwas wasdropping. dropping. D/I degree of ORGO was incredibly high, created at 120 120 °C
Scheme 3. 3. Mechanism Mechanism of” of” Cut Cut off off “effect. “effect. Scheme
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Figure 5. Cont.
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Figure 5. AFM image of RGO ((b) ORGO at 35 ◦°C, 0.5 h; (c) ORGO at 35 ◦°C, 5 h; (d) ORGO at 50 ◦°C, Figure Figure5. 5. AFM AFMimage imageof ofRGO RGO((b) ((b)ORGO ORGOat at35 35°C, C,0.5 0.5h;h;(c) (c)ORGO ORGOatat35 35°C, C,55h;h;(d) (d)ORGO ORGOatat50 50°C, C, 0.5 h; (e) ORGO at 50 ◦°C, 5 h; (f) ORGO at 80 ◦°C, 0.5 h; (g) ORGO at 80 ◦°C, 5 h; (h) ORGO at 120 ◦°C, 0.5 0.5h; h;(e) (e)ORGO ORGOat at50 50°C, C,55h; h;(f) (f)ORGO ORGOat at80 80°C, C,0.5 0.5h; h;(g) (g)ORGO ORGOat at80 80°C, C,55h;h;(h) (h)ORGO ORGOatat120 120°C, C, 0.5 h; (i) ORGO at 120 ◦°C, 5 h). 0.5 0.5h; h;(i) (i)ORGO ORGOat at120 120°C, C,55h). h).
◦
◦
Figure spectroscopy of of GO GO and and RGO RGO (a: (a: GO; GO;b: b:ORGO ORGOatat35 35°C,C,0.5 0.5h;h;c:c:ORGO ORGOatat3535°C,C, Figure 6. 6. Raman Raman spectroscopy 5 Figure 6. Raman spectroscopy GO and RGO (a: GO; b: ORGO at 35 °C, 0.5 h; c: ORGO at 35◦ °C, 5 ◦ C, 0.5 h; e:ofORGO ◦ C, ◦ C, 5h;h;d:d:ORGO ORGOatat5050°C, at 50 5 h; f: ORGO at 80 0.5 h; g: ORGO at 80 C, 5 h; 0.5 h; e: ORGO at 50 °C, 5 h; f: ORGO at 80 °C, 0.5 h; g: ORGO at 80 °C, 5 h; h: h; d: ORGO at 50◦ °C, 0.5 h; e: ORGO at 50 ◦°C, 5 h; f: ORGO at 80 °C, 0.5 h; g: ORGO at 80 °C, 5 h; h: h: ORGO 120 i: ORGO at 120 5 h). ORGO at at 120 °C, C, 0.50.5 h; h; i: ORGO at 120 °C, C, 5 h). ORGO at 120 °C, 0.5 h; i: ORGO at 120 °C, 5 h). Table 3. Fitting parameters describing the ID /IG for the ORGO shown in Figure 5. Table 3. Fitting parameters describing the ID/IG for the ORGO shown in Figure 5. Table 3. Fitting parameters describing the ID/IG for the ORGO shown in Figure 5. No. a b c d e f g h i
No. a b c d e f g h i a b 2.26 1.94 c1.77 d1.95 e1.80 f 1.79 g 1.92 h 2.13 i 1.75 ID/IG 2.26 1.94 1.77 1.95 1.80 1.79 1.92 2.13 1.75 ID/IG 2.26 1.94 1.77 1.95 1.80 1.79 1.92 2.13 1.75
IDNo. /IG
Ultimately, Ultimately, we we constructed constructed a model model for for the the evolution evolution of of the the composition, composition, structure structure and and Ultimately, we constructed a model for the evolution of the composition, structure and morphology of RGO RGO as asthe thereaction reactiontimes timesand andreaction reaction temperatures changed (Scheme As can we temperatures changed (Scheme 4). 4). As we morphology of RGO as the reaction times and reaction temperatures changed (Scheme 4). As we can can seeScheme in Scheme 4, with a low temperature short time, the reduction reaction occurred see in 4, with a low temperature and and short time, the reduction reaction occurred first;first; the see in Scheme 4, with a low temperature and short time, the reduction reaction occurred first; the the oxygen–containing functional group significantly decreasedwith withrestoration restoration of of the graphitic oxygen–containing functional group significantly decreased graphitic oxygen–containing functional group significantly decreased with restoration of the graphitic structure. At At the the same same time, some grafting occurred, and and the the particle particle size size decreased. decreased. At this this structure. At the same time, some grafting occurred, and the particle size decreased. At this temperature, with a longer time more reduction reduction reaction reaction occurred; occurred; a slight slight decrease of the the oxygen oxygen temperature, with a longer time more reduction reaction occurred; a slight decrease of the oxygen group content was shown, while the grafting chain had a slight increase. Considering the effect of the group content was shown, while the grafting chain had a slight increase. Considering the effect of the temperature, 80 °C was a demarcation point, i.e., below 80 °C, only the reduction reaction occurred, temperature, 80 °C was a demarcation point, i.e., below 80 °C, only the reduction reaction occurred,
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group while the grafting chain had a slight increase. Considering the11effect Materialscontent 2018, 11, xwas FOR shown, PEER REVIEW of 15 of the temperature, 80 ◦ C was a demarcation point, i.e., below 80 ◦ C, only the reduction reaction ◦ C, the ◦ C with and aboveand 80 °C, the 80 reaction ofreaction edge grafting was activated. 80 °C with a short timea (0.5 h),time the occurred, above of edge grafting was At activated. At 80 short increase of increase grafting of chain was still not obvious, the reaction rate of edge grafting was still much (0.5 h), the grafting chain was still notsince obvious, since the reaction rate of edge grafting was lower thanlower reduction reaction. The reduction degree also increased with thewith temperature, which still much than reduction reaction. The reduction degree also increased the temperature, ◦ finallyfinally resulted in theinstacking and and agglomeration of the RGO sheets at at 120 which resulted the stacking agglomeration of the RGO sheets 120°CCfor for55h. h. This This model dispersion result result of of different different RGO RGO dispersions. dispersions. could explain the exfoliation and dispersion
Scheme 4. The evolution of RGO under different reaction conditions.
3.3. Dispersion Stability Stability of 3.3. Dispersion of ORGOs ORGOs As mentionedin inthe theExperimental ExperimentalSection, Section, as–prepared reduced graphite oxide material As mentioned thethe as–prepared reduced graphite oxide material was − 1 was dispersed in THF a nominal concentration 2 mg·−1 mL theofaid of bath ultrasonication, −1 dispersed in THF at a at nominal concentration of 2 of mg·mL with with the aid bath ultrasonication, and and the dispersions were then allowed to settle for several weeks. Figure 7 shows digital pictures of the dispersions were then allowed to settle for several weeks. Figure 7 shows digital pictures of the the dispersions ForFor thethe justjust sonicated samples, all the graphite oxide dispersions three threeweeks weeksafter aftersonication. sonication. sonicated samples, allreduced the reduced graphite was in THF. manymany of these dispersions displayed only only short–term stability and oxidedispersed was dispersed in However, THF. However, of these dispersions displayed short–term stability almost completely precipitated in a matter of hours or a few days (Figure 7b–d). In contrast, suspension and almost completely precipitated in a matter of hours or a few days (Figure 7b–d). In contrast, of ORGO under intenseunder conditions (Figure 7e–g) exhibited long–term stability, which showed a visually suspension of ORGO intense conditions (Figure 7e–g) exhibited long–term stability, which ◦ C for 5 h showed a deep color, the suspension was deep color. Although dispersion of ORGO at 120 showed a visually deep color. Although dispersion of ORGO at 120 °C for 5 h showed a deep color, opaque with plenty of flocs. with plenty of flocs. the suspension was opaque
3535 °C,◦ C, 0.50.5 h; h; (c) (c) ORGO at 35 h;5(d) Figure 7. Digital Digital pictures picturesof ofORGO ORGOand andHRGO. HRGO.((b) ((b)ORGO ORGOatat ORGO at °C, 35 ◦5C, h; ◦ ◦ ◦ ORGO at 50 at 50 h;5(f) at 80 at 80 h;5(h) (d) ORGO at °C, 50 0.5 C, h; 0.5(e) h; ORGO (e) ORGO at °C, 50 5C, h; ORGO (f) ORGO at°C, 80 0.5 C, h; 0.5(g) h; ORGO (g) ORGO at°C, 80 ◦5C, h; ORGO at 120 °C, ◦0.5 at 120 °C, ◦5C, h;5(j) (80 (80 °C, ◦5C, h)). (h) ORGO at 120 C, h; 0.5(i) h;ORGO (i) ORGO at 120 h;HRGO, (j) HRGO, 5 h)).
Dispersion performance of of ORGOs ORGOs were were also also confirmed confirmed by by the the UV–vis UV–vis absorption absorption spectra. spectra. Dispersion performance Figure absorption spectra spectrum of reduced graphene graphene Figure 88 shows shows the the UV–vis UV–vis absorption spectra of of RGO. RGO. The The UV–vis UV–vis spectrum of reduced oxide exhibited two characteristic features that can be used for identification: A maximum at 231 nm, oxide exhibited two characteristic features that can be which corresponds to the π*π transitions of aromatic C=C bonds, and a shoulder at ~380 nm, which can which corresponds to the π*π transitions of aromatic C=C bonds, and a shoulder at ~380 nm, which be to the transitions of the bonds [30]. [30]. SinceSince the absorption peakpeak at 200 canattributed be attributed to n–π the n–π transitions ofC=O the C=O bonds the absorption atnm 200was nm was interfered by THF, while C=O bonds were stable under different reaction conditions, so we chose the absorption at 300–400 nm as a reference. For the different RGO samples, we noticed (Figure 8) that the RGO created at 120 °C for 5 h had the highest absorption intensity, but the suspending particles were flocs. This was closely followed by the curve changed, of the samples in Figure 8g,h,
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interfered by THF, while C=O bonds were stable under different reaction conditions, so we chose the absorption nm REVIEW as a reference. For the different RGO samples, we noticed (Figure 8) that Materials 2018,at 11,300–400 x FOR PEER 12 ofthe 15 ◦ RGO created at 120 C for 5 h had the highest absorption intensity, but the suspending particles were flocs. was closely the curveperformance. changed, of the in Figure 8g,h, exhibited whichThis exhibited veryfollowed similar by dispersion Atsamples last, Figure 8b–f hadwhich notably poor very similar dispersion performance. At last, Figure 8b–f had notably poor dispersion stability. dispersion stability.
◦ C,0.5 ◦ C, absorption spectra spectra of of RGO RGOand andHGO HGO(b: (b:ORGO ORGOatat35 35°C, 0.5h;h;c:c:ORGO ORGOatat3535 Figure 8. UV–vis absorption °C, 5 ◦ C, 5 h;d:d:ORGO ORGO ORGO 5 h; f: ORGO h; ORGO g: ORGO at 80 h; h; atat 5050 °C, 0.50.5 h; h; e: e: ORGO at at 5050 °C,◦ C, 5 h; f: ORGO at at 80 80 °C,◦ C, 0.50.5 h; g: at 80 °C,◦ 5C,h;5 h: h: ORGO at 120 h;ORGO i: ORGO at 120 5 h). ORGO at 120 °C,◦ C, 0.50.5 h; i: at 120 °C,◦5C,h).
It is acknowledged that that GO GO could could disperse disperse in in polar polar solvent solvent like like water water and and DMF DMF [31]. [31]. It is generally generally acknowledged On the other ability in in medium medium polar polar solvent solvent like like THF, THF, while On the other hand, hand, GO GO had had limited limited dispersing dispersing ability while the the as-prepared ORGO could be easily dispersed in THF. Such dispersions are termed lyophobic colloids as-prepared ORGO could be easily dispersed in THF. Such dispersions are termed lyophobic colloids and often interpreted interpreted by by the the theory theory of of Derjaguin, Derjaguin, Landau, Landau, Verwey, Verwey, and and often and Overbeek Overbeek (DLVO) (DLVO) [32]. [32]. In In the the DLVO DLVO theory theory of of lyophobic lyophobic colloids, colloids, there there are are three three repulsive repulsive forces forces which which can can counteract counteract attractive attractive interparticle the case case of of graphene) graphene) and, and, if if sufficient sufficient in in magnitude, magnitude, render render dispersions dispersions interparticle forces forces (vdWs (vdWs in in the stable for an an extended extended period period of of time. time. These These are are (1) (1) electrostatic, electrostatic, where where the the presence presence of of surface surface charge charge stable for establishes an electrical double layer (EDL) around the colloidal particles and results in an electrostatic establishes an electrical double layer (EDL) around the colloidal particles and results in an repulsion between approaching particles, (2) particles, steric, where bulky surface groups and solvent electrostatic repulsion between approaching (2) steric, where bulky surface groupseffect and physically prevent the close approach of particles, (3) Brownian movement, improve the solvent effect physically prevent the close approachand of particles, and (3) Brownianwhich movement, which kinetics stability of lyophobic colloids. In our work, two key factors of grafting chains and reduction improve the kinetics stability of lyophobic colloids. In our work, two key factors of grafting chains degree worked against other resulting in the final dispersing properties of ORGOs.properties For instance, and reduction degree each worked against each other resulting in the final dispersing of the grafting on ORGO increasechains steric on hindrance and improved solvent effect ORGOs. Forchains instance, the grafting ORGO physically, increase steric hindrancethe physically, and according andaccording Hansen solubility parameters theory [33] (specificparameters analysis was supplied improved to theHildebrand solvent effect to Hildebrand and Hansen solubility theory [33] in Supplementary Materials, Table On the opposite, the high degree reduction will increase the (specific analysis was supplied inS1). Supplementary Materials, Table S1).ofOn the opposite, the high vdWs between different sheets, which breaks the stability of ORGO sheets in solvent. Additionally, degree of reduction will increase the vdWs between different sheets, which breaks the stability of the intense Brownian movement of ORGO with small particle size will improve kinetics ORGO sheets in solvent. Additionally, the intense Brownian movement of ORGOthe with smallstability particle ◦ ◦ C for 0.5 h as assistant. It was not hard to understand why ORGO atto 80understand C for 5 h and size will improve the kinetics stability as assistant. It wascreated not hard why120 ORGO created has the best dispersion stability, since they had enlarged edge chains, and limited reduction degree. at 80 °C for 5 h and 120 °C for 0.5 h has the best dispersion stability, since they had enlarged edge ◦ The samples created at temperatures lower than 80 C hadatpoor suspension stability, since theypoor did chains, and limited reduction degree. The samples created temperatures lower than 80 °C had ◦ C for 5 h, the dispersing ability was not have enough grafting Fornot thehave sample created at 120chains. suspension stability, sincechains. they did enough grafting For sample created at 120 poor the strong Van der forces due to the highVan degree of reduced graphene. °C forbecause 5 h, the of dispersing ability wasWaals’ poor because of the strong der Waals’ forces due to the high degree of reduced graphene. 3.4. Heat Resistant Properties of ORGOs 3.4. Heat Resistant Properties of ORGOs Thermogravimetric Analysis was performed to evaluate the structural recovery extent of ORGO compared to graphene. This characterization was since all of the excellent properties of Thermogravimetric Analysis was performed tonecessary evaluate the structural recovery extent of ORGO compared to graphene. This characterization was necessary since all of the excellent properties of graphene were closely relevant to its graphitic structure. Figures 9 and 10 presented the TGA curves for graphene oxide and reduced graphene oxide, which showed the mass loss during heating in air. Graphene oxide has mass losses in two temperature ranges. The first one, between 120 °C and 300 °C, corresponded to the removal of the oxygen–containing functional groups. The second one
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graphene were closely relevant to its graphitic structure. Figures 9 and 10 presented the TGA curves for graphene oxide and reduced graphene oxide, which showed the mass loss during heating in air. Materials 2018, 11, x FOR PEER REVIEW 13 of◦ 15 Graphene oxide has mass losses in two temperature ranges. The first one, between 120 ◦ C and 300 C, Materials 2018, 11, x FOR PEER REVIEW 13 of 15 corresponded to the removal of the oxygen–containing functional groups. The second one occurred at occurred at approximately 500 °C due to the bulk pyrolysis of the carbon skeleton [34]. The curves approximately 500 ◦ C due to500 the°C bulk of the carbon skeleton [34]. The curves indicated that occurred approximately duepyrolysis to the bulk pyrolysis the carbon [34]. The indicated at that below 120 °C, the stability slightly increasedofwith higher skeleton temperatures. Thecurves RGO ◦ ◦C below 120 that C, the stability slightly increased with higher temperatures. Thetemperatures. RGO createdThe at 120 indicated below 120exhibit °C, the stability slightly with RGO created at 120 °C did not a substantial weightincreased loss before 300 higher °C, while it had a drastic weight ◦ ◦ did not at exhibit a did substantial weight loss before 300 loss C, while it had°C, a drastic loss at 300 C. created 120 not exhibit a substantial weight before while itweight hadand a drastic weight loss at 300 °C.°C The TGA performance was associated with the 300 reduction degree particle size. The TGA performance wasperformance associated with reductionwith degree particledegree size. Since reduction loss 300 °C. The TGA wasthe associated the and reduction and the particle size. Sinceatthe reduction degree of◦ RGO created at 120 °C was high, few oxygen–containing functional degree of RGO created at 120 C was high, few oxygen–containing functional groups were removed Since reduction degree of 300 RGO at 120 was high, fewsmallest, oxygen–containing functional groupsthe were removed before °C.created However, its °C particle size was which resulted in the ◦ C. before 300 However, its particle size was smallest, which resulted in the drastic weight loss at groups were removed before 300 °C. However, its particle size was smallest, which resulted in the drastic weight loss at 300 °C. The TGA results for RGO produced with different times and a certain ◦ 300 C. weight The TGA results for°C. RGO withfor different times and with a certain temperature were shown drastic loss at 300 Theproduced TGA RGO different timesincreased and a certain temperature were shown in Figure 9. It results was obvious thatproduced the thermostability slightly with in Figure 9. It was obvious that the thermostability slightly increased with longer times. These results temperature were shown in Figure 9. It was obvious that the thermostability slightly increased with longer times. These results agreed with the XPS spectra. The TGA result demonstrated that ORGO agreed with the XPSresults spectra. The TGA result demonstrated that created at mildthat condition longer These agreed with the XPS spectra. The properties TGAORGO resultasdemonstrated createdtimes. at mild condition held almost the same heat resisted ORGO created at ORGO higher held almost the same heat resisted properties as ORGO createdproperties at higher temperature and longer time created at mild condition held almost the same heat resisted as ORGO created at higher temperature and longer time (Shown in Figure 10), which confirmed that the mild condition could (Shown in Figure 10), which confirmed that the mild condition could be used to lower the cost of temperature and the longer (Shown in Figure 10), which confirmed that the mild condition could be used to lower costtime of bulk production. bulk production. be used to lower the cost of bulk production.
Figure 9. TGA of GO and RGO created at different temperatures. Figure 9. TGA of GO and RGO created at different temperatures.
Figure 10. 10. TGA TGA of of RGO RGO created created with with different Figure different reaction reaction times. times. Figure 10. TGA of RGO created with different reaction times.
4. Summary and Conclusions 4. Summary and Conclusions 4. Summary and Conclusions In conclusion, the reduction degree and grafting extent vary under different reaction conditions In conclusion, the reduction degree and grafting extent vary under different reaction conditions sinceIn the reaction mechanism changes. The reaction initiated by the nucleophilic reaction conclusion, the reduction degreeThe andreduction grafting extent vary under different reaction conditions since the reaction mechanism changes. reduction reaction initiated by the nucleophilic reaction since the reaction mechanism changes. The reduction reaction initiated by the nucleophilic reaction between amine and epoxy group occurs easily, i.e., a high ratio of C–C signal was discovered in the between amine epoxy group occurs easily, i.e.,slightly a high ratio of C–C signal was time discovered in the XPS results at 35and °C for 0.5 h. The reduction degree increased with a longer and a higher XPS results at 35 °C for 0.5 h. The reduction degree slightly increased with a longer time and a higher temperature. However, the edge grafting reaction between amine and carboxyl group required temperature. However, the edgeofgrafting reaction structure between and amine and carboxyl group intense condition. The evolution the composition, morphology of the RGO required resulted intense condition. The evolution of the composition, structure and morphology of the RGO resulted
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between amine and epoxy group occurs easily, i.e., a high ratio of C–C signal was discovered in the XPS results at 35 ◦ C for 0.5 h. The reduction degree slightly increased with a longer time and a higher temperature. However, the edge grafting reaction between amine and carboxyl group required intense condition. The evolution of the composition, structure and morphology of the RGO resulted in the different dispersion performances and heat resistant properties of RGO. The RGO created at 80 ◦ C for 5 h and 120 ◦ C for 0.5 h has the best dispersing stability since they have more edge grafting chains, small sheet size and suitable reduction degree. To sum up, the modification of ODA on GO can be controlled and the product can be designed based on the above conclusions. It is facilitated for the practical use of graphene: The high degree of reduction under mild condition (even closed to room temperature) was attractive for the industrial production of graphene-based composite/hybrid. The ORGO product with both a high reduced degree and a large number of grafting chains can disperse well in THF, and can prepare a resin-based composite with both high physical properties and tribological properties. Besides, the amine-modified GO could be used as an additive in lubricating oil and anti-corrosion additives in coatings. These investigative works. have been conducted and excellent results have been achieved. We will summarize and report them soon. Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/11/9/1710/ s1, Figure S1: The characterization of original material of GO (from left to right: XRD of GO; TEM of GO and AFM of GO), Figure S2: FTIR spectrum of ODA, Figure S3: TG of GO and RGO (35 ◦ C, 0.5 h), Figure S4: AFM and digital picture of ORGO (35 ◦ C, 0.5 h), Figure S5: Deconvolution of the peaks curves of C 1s and N 1s spectrum of ORGO created at 35 ◦ C for 24 h, Figure S6: Deconvolution of the peaks curves of C 1s spectrum of HRGO (labelled j) created at 80 ◦ C for 5 h, Table S1: Hansen Solubility Parameters for Various solvents. Author Contributions: Investigation, T.B.; Methodology, Y.W.; Resources, Y.Z. and X.Y.; Writing—review & editing, Z.W. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflicts of interest.
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