Yang Liuab,
Cheng-Lu Lianga,
Jing-jie Wub,
Rui-Ying Baoa,
Guo-Qiang Qia,
Yu Wangc,
Wei Yang*a,
Bang-Hu Xiea and
Ming-Bo Yanga
aCollege of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, 610065, Sichuan, China. E-mail: weiyang@scu.edu.cn
bDepartment of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA
cSchool of Mechanical and Materials Engineering, Washington State University, Pullman, 99164, USA
First published on 11th March 2016
As a promising material with broad applications, reduced graphite oxide (rGO) hydrogels have attracted more and more great attention recently. However, most reports on rGO hydrogels focused on their applications, while the formation mechanism has not been paid enough attention. For the first time, we demonstrated the higher the ability of the solvents to form hydrogen bonds with the rGO sheets, the better the structural stability and properties of gel are. This study indicates that hydrogen bonding between solvent molecules and the oxygen-containing functional groups on rGO sheets is vital to achieve high-performance gels.
In spite of the increasingly great potential applications of reduced graphite oxide (rGO) gels,15–19 little attention has been paid to the formation mechanism. Xu and coworkers obtained 3D self-assembled graphene hydrogel by hydrothermal process and conjectured that the combination of hydrophobic and π–π interactions between rGO sheets caused the 3D random stacking of flexible graphene sheets.20 Although this viewpoint has been widely accepted,21–25 there is no strong evidence clearly showing these interactions so far. However, to the best of our knowledge, hydrogen bonding is a crucial interaction during the formation of GO composites26 and rGO gel27,28 and revealing the formation mechanism of rGO gel is vital for preparing higher quality rGO gels with controllable microstructures and functions.
In this study, the role of the ability of solvent to form hydrogen bonds with the oxygen-containing functional groups on rGO sheets in controlling the structures and properties of gel has been investigated. The microstructure and properties of as-prepared gels were characterized in detail. It was found that the rGO gels which were formed by stronger hydrogen bonding perform much more perfect gel network, better structural stability and higher electrical conductivity. These findings experimentally confirm that the hydrogen bond between the solvents and residual oxygen-containing groups of rGO plays an important role in the self-assembly of rGO gel for the first time.
Freeze-dried GO and rGO aerogels which kept their primary structures in the gels were characterized by the following methods. The morphologies of the samples were characterized using an INSPECT F SEM and TEM (Tecnai G2F20 S-TWIN, FEI, USA). XPS measurements were performed on an XSAM 800 instrument (Kratos Company, UK) with AlKα radiation (hν = 1486.6 eV) and XPSpeak 41 software (Chemistry, CUHK) was used to calculate the atomic concentrations. FTIR was performed over the wave number range of 4000–400 cm−1 using a Nicolet 6700 FTIR spectrometer (Nicolet Instrument Company, USA).
Solvent | Water | EG | DMF | NMP |
δH | 42.3 | 26.0 | 11.3 | 7.2 |
According to the value of hydrogen bonding cohesion parameter (δH), water shows the highest hydrogen bonding ability with the residual oxygen-containing groups on rGO sheets after the reduction of GO by VC, while NMP shows the lowest among these four solvents. The ability to form hydrogen bonds with the oxygen-containing functional groups on rGO sheets for EG and DMF is between that of water and NMP. As a consequence, the hydrogen bonding effect in water–rGO system shows the highest strength while the hydrogen bonding effect in NMP–rGO shows the lowest strength. Different degree of structural stability and mechanical strength in the as formed rGO gels were obtained in these four solvents due to the different degree of hydrogen bonding effect in Fig. 1(b). The hydrogel network of rGO–H2O hydrogel is strong enough to form a solid-like cylinder suspended in solution. Although a hydrogen bond network also formed in rGO–EG, the strength of the gel is not very high and the cylinder can be easily broken. In contrast, the rGO sheets in rGO–DMF and rGO–NMP didn't form any stable gel network. The photos of samples indicate the formation of some network related to hydrogen bonding.
Similarly, the cross-linked network of rGO–H2O hydrogel can be directly seen in microstructure of gels by scanning electron microscope (SEM) and transmission electron microscopy (TEM) images (Fig. 1(c)) and more SEM images of four gels are shown in ESI (Fig. S2†). As for rGO–H2O, a porous network of rGO–H2O hydrogel could be seen clearly. The porous network was formed from the overlapping of graphene sheets which was shown in the amplification of image of rGO–H2O. Then rGO–H2O sample was ultramicrotomed perpendicular to the graphite sheets, so we can see the joints between layers in TEM images as well. Compared with rGO–H2O, the porous structure of rGO–EG was not formed completely and the gel network was not tight as that of rGO–H2O. However, there is no obvious porous structure in rGO–DMF and rGO–NMP gel. The SEM images of rGO–DMF and rGO–NMP show a smooth surface instead of a porous network structure indicating that the rGO gel network was not formed in DMF and NMP. The results of SEM and TEM images for these different gels are consistent with the photos of four as-prepared gels. These results indicate that rGO sheets can form a stronger 3D gel network in solvents which show higher hydrogen bonding ability.
The construction scheme of rGO gel can be understood as shown in Scheme 1. The self-assembly process is driven by the hydrogen bond between solvents and the residual oxygen-containing groups of rGO which can be confirmed by XPS measurements (Table 2) and dehydroascorbic acid (DHA), the oxidation product of VC.34 At the beginning of the self-assembly of rGO gels, GO sheets were suspended uniformly in different solvents by hydrophilic interaction.35 After the addition of VC to GO suspension, the reduction of GO occurred. The sp3 hybridization C–C bond was reduced to sp2 hybridization CC bond. The spatial structures of CC sp2 hybridization structure which were π–π conjugated are flat. The large-area flat structure of rGO sheets were crimped and overlapped by hydrogen bonds between residual oxygen-containing groups of rGO with water and DHA. So an integral self-assembly network of rGO–H2O hydrogel was formed successfully. As for rGO–EG, the gel network of that was constructed by hydrogen bond between hydroxyl of EG with residual oxygen-containing groups of rGO, but the strength of rGO–EG gel network is lower than that of rGO–H2O because the δH value of EG is lower than H2O. In contrast, the rGO gel networks in DMF and NMP hardly formed because the ability of DMF and NMP to form hydrogen bond with the residual oxygen-containing groups of rGO is too weak to obtain gel networks. The flexible rGO sheets stacked spontaneously and formed wrinkled rGO aggregates in DMF and NMP as shown in Fig. 1(c). Therefore, the properties and microstructures of these gels further confirm that the hydrogen bonding is an important factor during the formation of rGO gels.
Scheme 1 The formation of self-assembled 3D network for rGO gels during the reduction of GO by VC in various solvents. |
Sample | Pristine G | GO | rGO–H2O | rGO–EG | rGO–DMF | rGO–NMP |
C/O | 119 | 2.4 | 4.0 | 4.0 | 3.5 | 4.3 |
In order to confirm the existence of residual oxygen-containing groups and eliminate the effect of reduction degree of GO on the gel stability, the reduction degree of GO for the four gels was characterized by X-ray photoelectron spectroscopy (XPS, Table 2 and Fig. S3†) and Fourier-transform infrared spectroscopy (FTIR, Fig. S4 in ESI†), Raman (Fig. S5 and Table S1 in ESI†). The samples were freeze-dried before characterizations. After freeze-drying, the rGO–H2O and rGO–EG aerogel become brittle because of the removal of solvents wrapped in rGO sheets, which destroyed the integrity of the hydrogen bond network. The phenomenon also indicates that hydrogen bonding is the crucial interaction during the self-assembly of rGO gels.
Table 2 shows the C/O ratio value of pristine G, GO and the as-prepared rGO gels reduced in different solvents. The C/O ratio values of all the four rGO gels are similar and increase markedly after the reduction of GO. It indicates an efficient deoxidization of GO and the reduction degree of rGO gels in different solvents are nearly the same. So the formation of a strong gel network in water and EG is not determined by a higher reduction degree of GO but rather the ability to form hydrogen bond between solvents and the oxygen-containing functional groups on rGO sheets. Otherwise, the C/O ratio values of rGO aerogels are far lower than that of pristine G. It demonstrates that there were a number of residual oxygen-containing groups on rGO sheets which is confirmed by C1s XPS spectra (Fig. S4†) as well. So hydrogen bonds between the residual oxygen-containing groups of rGO with solvents and DHA are not only existent but also the key to achieve a more integrate gel network and higher-performance gels.
The mechanical strength and structural stability of the gels could further confirm the different degree of gel network of as-prepared gels in different solvent. In Fig. 2(a) and (b), the mechanical strength of the four rGO gels with time was tested by dynamic time sweep. The value of G′ (storage modulus) for rGO–H2O hydrogel is almost three orders of magnitude higher than the second high G′ value of rGO–EG gel, indicating that the rGO–H2O hydrogel network is the strongest among the four gels. And the weaker network of rGO–DMF and rGO–NMP gels than rGO–H2O and rGO–EG resulted in lower G′ value of that as expected. The values of G′ for the four gels were higher than that of the G′′ (loss modulus), also indicating the formation of gel network.36,37 The correlation between the hydrogen bonding cohesion parameters and the structural stability and mechanical strength of different gels (in Fig. 2(c)) revealed that hydrogen bonding actuate the self-assembly of rGO gels.
A stable self-assembled network of rGO gels will lead to a stable electric conductive pathway,38 so the electrical resistances of the gels were also measured during time sweep as shown in Fig. 2(d). The value of electrical resistance of rGO–H2O is 105 times lower than that of water, indicating that the hydrogel network of rGO–H2O provided a conductive pathway for electron even the pure water will impede the transportation of electron. In contrast, the electrical resistance of rGO–EG, rGO–DMF and rGO–NMP gels are much higher than that of rGO–H2O hydrogel. The electrical conductivity of rGO–DMF and rGO–NMP are similar and lower than that of rGO–EG. It is revealed that the gel network of rGO–DMF and rGO–NMP is less integral than that of rGO–EG gel and rGO–H2O hydrogel. The results of electrical resistance of the gels also indicate that the higher ability to form hydrogen bond of solvents, such as EG and water, the more integral network of rGO gels for electron conduction.
Footnote |
† Electronic supplementary information (ESI) available: More SEM images of gels prepared in different solvent, XPS, FT-IR and Raman analysis of the reduction degree of GO. See DOI: 10.1039/c6ra02942f |
This journal is © The Royal Society of Chemistry 2016 |