Contrasting Magnetic Properties of Thermally and Chemically

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Contrasting Magnetic Properties of Thermally and Chemically Reduced Graphene Oxide Kousik Bagani, Mayukh K Ray, Biswarup Satpati, Nihar Ranjan Ray, Manas Sardar, and Sangam Banerjee J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 May 2014 Downloaded from http://pubs.acs.org on May 28, 2014

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The Journal of Physical Chemistry

Contrasting Magnetic Properties of Thermally and Chemically Reduced Graphene Oxide Kousik Bagani†, Mayukh K. Ray†, Biswarup Satpati†, Nihar R. Ray†, Manas Sardar‡ and Sangam Banerjee†,⃰ †

‡

Surface Physics Division, Saha Institute of Nuclear Physics, Kolkata- 700064, India.

Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam- 603102, India.

* Corresponding author’s email. [email protected]

ABSTRACT: In this paper we report the observation of enhanced magnetization in graphene oxide (GO) after thermal annealing. We have also proposed that this enhancement in magnetization is due to the increased density of zigzag edges. We conjecture that on annealing, the random epoxy groups in the native GO migrates over the GO surface by acquiring thermal energy and self assemble to form several long chains of epoxy groups. Subsequently, upon thermal reduction the GO sheet is unzipped along these long chains giving rise to more zigzag edges resulting in enhancement of magnetization. We also found out that the density of epoxy groups plays an important role in the unzipping process. If the density of epoxy groups are low, then unzipping of GO is not possible. Chemical reduction of GO does not favor unzipping.

KEYWORDS:

Graphene, graphene oxide, thermal and chemical reduction, magnetism,

coercivity.

ACS Paragon Plus Environment

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1 INTRODUCTION

One of the most well studied routes for the preparation of single layer graphite (graphene) is the oxidation and reduction process. The most cost effective process to obtain graphene is the exfoliation of natural graphite layers after oxidation to get individual oxidized layers and then deoxygenation (reduction) of these individual layers.1-6 During the oxidation process, various functionalized groups (-OH, -O-, -COOH, C=O etc.) goes into the graphene skeleton breaking the π bond of graphene structure,7 but the exact decoration of the functionalized groups on the graphene skeleton is still unknown. NMR study of graphene oxide shows that carbonyl groups are located in the periphery of the graphene oxide sheet8, 9. Therefore, only hydroxyl (-OH) and epoxy (-O-) groups are abundant in the interior region of the graphene sheets. Several groups have theoretically and experimentally shown that the epoxy groups self assembles in a line in the graphene structure10-12. Few experimental groups have tried to cut graphene sheets into nano size, by exploiting this tendency of the epoxy groups assembling in a line and further reducing it.12, 13 Also in a similar fashion, graphene nano ribbon is prepared from the carbon nanotube.14, 15 There are two possibilities of formation of epoxy chain in a graphene oxide sheet: (a) adjacent epoxy groups are bonding on the opposite sides of the hexagonal rings forming a line or (b) adjacent epoxy groups are bonding with alternate C-C bonds of graphene forming a line as shown schematically in Figure 1a. When the epoxy group bonds with C-C bond along a line then it has been shown16 that C-C bond breaks and forms C-O-C ether structure. If this GO sheet is reduced then due to release of oxygen, the sheet unzips to form either zigzag or armchair edges in these two cases respectively. It is energetically favorable to place the epoxy groups on the opposite side of the hexagonal ring (i.e. 1st case) and it is theoretically shown that these epoxy chains are responsible for the oxidative cutting of graphene.11, 16-18 But there is no conclusive proof of the


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generation of zigzag edges after reduction as predicted by the theory. Using photoluminescence spectra, D. Pan et al.13 showed that after hydrothermal reduction (cutting) of graphene sheets, the number of zigzag edges increases. It is also well known that zigzag edges and structural defects (vacancies, adatoms etc.,) are responsible for unusual magnetic properties of carbon based graphene related materials.19-24 In this paper we have focused on the edges and we feel that the zigzag edges play an important role in the magnetic property of graphene. This is because at the zigzag edges, the π electrons are energetically degenerate (non dispersive) and have highly localized edge states. These edge states are populated with the same spin to minimize the coulomb repulsion energy leading to large moments at the zigzag edge boundary.19,

20, 25-29

Therefore, unzipping (cutting) of graphene oxide sheets which creates zigzag edges is expected to enhance the magnetic moment. Experimentally, the optimal procedure i.e. either chemical or thermal annealing to maximize the magnetic moment of graphene sheets is unclear at present. We have tried to address this issue in this paper. In this paper with the help of magnetic property studies, we have tried to establish that thermal reduction favours the unzipping processes leading to enhancement of exposed zigzag edges and in contrast the chemical reduction does not favour unzipping process. We have used magnetization measurements to probe the emerging moments after chemical or thermal treatment of graphene oxide. 2 EXPERIMENTAL DETAILS

We have prepared graphene oxide (GO) from natural graphite by chemical route (Hummers method)

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and partially reduced it by hydrazine (RGO). To observe the role of epoxy groups in

unzipping of graphene, we have annealed two types of graphene oxide (a) as prepared graphene oxide (GO) which has high density of epoxy groups in its structure and (b) partially reduced


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graphene oxide (RGO) where epoxy groups are much less than that in the as prepared GO. We annealed both GO and RGO at 600oC in Ar atmosphere for two hours and those are labeled as GO-600 and RGO-600 respectively. Transmission electron microscopy (TEM) (FEI, Tecni F30, operating at 300 keV) and Raman spectroscopy of all the samples were done to characterize the samples. Magnetic measurements were carried out in a SQUID magnetometer MPMS XL 7, Quantum Design. ZFC-FC magnetization vs. temperature data from 10 K to 300K was taken during warming the samples in presence of 100 Oe field. Magnetic hysteresis curve was taken at three different temperatures (10K, 100K and 300K). 3 RESULTS AND DISCUSSIONS

In Figure 2 (a) and (b) we show the high angle annular dark-field scanning/transmission electron microscopy (STEM-HAADF) images of GO and RGO respectively. Fig. 2(c) and (d) depicts the TEM and high-resolution TEM image respectively of a single layer RGO-600 sheet. Fourier filtered image in the inset of Figure 2(d) from a region marked by a dotted box clearly shows the crystalline nature of the film. From the STEM-HAADF images we can see that the short range crumpling/winkles present in the GO sheet decreases upon chemical reduction. This is expected because random attachment of oxygen containing groups on the graphene surface leads to wrinkles in the structure12,

31-32

. Therefore, less wrinkles is observed after chemical

reduction of GO as we have reported earlier33. Figure 3 shows the Raman spectroscopy of all the samples. The so called G peak near 1590 cm-1 and D peak near 1300 cm-1 in the Raman data indicates that all the samples are graphene containing defects such as oxygen containing groups, atomic vacancy etc. The G peak is the characteristics of the sp2 hybridized carbon atom in the hexagonal ring and is due to the σ bond stretching modes of all pairs of sp2 atom.34, 35 The D peak arises due to the 2nd order process


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which requires a defect for activation.36-39 The GO sheets have both sp3 hybridized carbon region and unoxidized patches/fragments of sp2 hybridized carbon atoms. From the intensity ratio of the D peak and the G peak of Raman spectra, one can get an idea about the relative fraction of the sp2 patches. Average size of the sp2 hybridized region in the oxidized graphene sheet is inversely proportional to the intensity ratio of the D peak and the G peak (ID/IG).40 After partial reduction with hydrazine, large number of patches of sp2 carbon arises which are smaller in size that were present in GO.41 So average size of sp2 cluster decreases and hence ID/IG increases41-42 (see Figure 3c). After annealing the GO at 600oC for two hours the intensity of the D peak decreases (Figure 3b) which indicates that the GO-600 sample is less defective as we have explained below. Epoxy groups have an important role in the unzipping process. Since we have oxidized the graphite with highly oxidizing chemicals, we can assume that most of the epoxy groups are randomly attached on the graphene surface (see Figure 1b). T. Sun et al.18 have shown by DFT calculation that the epoxy groups can diffuse/migrate on the surface which could be responsible for nucleation and growth of extended linear arrangement of epoxy groups. We argue that during annealing, the randomly placed epoxy groups migrates on the GO surface by acquiring thermal energy and self organize into long chains of epoxy groups as shown schematically in Figure 1c. Further annealing leads to removal of epoxy groups along a line causing fracture and unzipping of graphene oxide sheets into fragments having zigzag edges as shown in Figure 1d. Thus, thermal reduction removes the oxygen and fragments the graphene sheets into pieces with exposed zigzag edges. So, GO-600 contains much less number of oxygen containing groups and larger number of zigzag edge which cannot enhance D peak.43-45 Therefore, after annealing GO at 600oC the intensity of D peak decreases. On the other hand we see that if we compare the Raman peaks of RGO and RGO-600, the D peak intensity does not decrease after thermal annealing (Figure 3d). After partial chemical


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reduction, large numbers of the epoxy groups are removed from the GO. Hence, the number of epoxy groups present in RGO may not be sufficient to organize themselves along long chains during annealing and hence preventing the unzipping and fragmentation of the sheet. This could be the reason for the comparable intensity of the D peak in RGO and RGO-600 sample. We will discuss the validity of this assumption when we analyze the magnetic measurement data. In Figure 4 we have shown the ZFC-FC magnetization versus temperature taken is a magnetic field of 100 Oe. It is clearly seen that the magnetic moment decreases after partial chemical reduction but increases after annealing in both GO and RGO. In our earlier studies33 we found that the chemical reduction process always leads to reduction of magnetic moment, in graphene oxide. We had proposed33 that apart from atomic vacancies and bigger topological defects, magnetic moment in graphene oxides can also originate from patches of -OH groups. The decrease in moment of graphene oxide after chemical reduction is essentially due to removal of such groups in the reduction process. On annealing GO and RGO the concentration of epoxy groups reduces further and the magnetic moment should decrease, but here we observe exactly the opposite behavior, i.e large increase in moment after annealing at 600oC in both GO and RGO. This shows that annealing leads to something more than mere removal of functional groups from graphene oxides. We can get further insight into this contradictory behavior by looking at the magnetic hysteresis (M-H loop) at different temperatures. In Figure 5 and Figure 6 we show the magnetic hysteresis curves of RGO and RGO-600 respectively. It is clearly seen that the magnetic coercivity increases with increasing temperature and the coercivity of RGO-600 is greater than that of RGO at all temperatures. We have argued that this anomalous increase of coercivity with increasing temperature33 is due to reduction/curing of wrinkles in the RGO sheet with increase in temperature. We see from TEM images that chemical reduction leads to lowering of wrinkles in the sheet. An increase in temperature also leads to reduction in


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wrinkles.33, 46 Reduced wrinkles has two important effects: (1) increased average local magnetic anisotropy energy density and (2) emergence of longer range magnetic interaction/ordering. Both these effects are explained as follows: Coercive field of a single domain magnetic particle is given by H C =

KV , where, KV is the magnetic anisotropy energy, K and V are the magnetic M

anisotropy energy density and volume of single domain respectively and M is the net magnetic r

moment of a single domain particle. For localized electrons, we can write KV = ∑i J iA where, r summation is over all the sites of a patch containing a net magnetic moment, A is local r

r

anisotropy axis and J iA is the single ion magnetic anisotropy energy at site i, Ei = J iA Si2, Ar . Origin of this is spin-orbit interaction. So, both magnitude of Ji and the local anisotropy direction r A depends sensitively on sp3 admixture into the local electronic orbital, due to local crumpling of

the structure. When there is large crumpling on the surface, the average (over many sites) value of KV is small. Crumpling is reduced at higher temperature and in materials with small number of functional groups on the surface of graphene. Therefore this leads to increase in average value of KV and hence increase in the coercivity. There is another additional mechanism for further increase in coercivity with decrease in wrinkle and increase in temperature. If we assume that in reduced graphene oxide there is finite non zero gaps in electronic energy levels, then with increase in temperature, the thermally activated electrons can mediate interaction (RKKY type)47 between isolated magnetic moments on reduced graphene oxide. Both of these effects lead to increase in coercivity after reduction, as well as, anomalous increase in coercivity with increase in temperature. Annealing RGO at 600oC removes many oxygen containing groups and this leads to lowering of wrinkles. We believe this could be the dominant reason for larger coercivity of RGO-600 compared to RGO at all temperatures. At T=10 K, RGO-600 has finite coercivity but


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RGO has almost no coercivity, behaving like a paramagnet. At a magnetic field of 2000 Oe, the magnetic moment of RGO is greater than RGO-600. This indicates that RGO has more paramagnetic moment than RGO-600 which seems reasonable because RGO contains more oxygen containing groups than RGO-600. As interaction between isolated moments in RGO-600 is stronger than RGO, the moments get aligned strongly in RGO-600 compared to RGO even at low magnetic fields (100 Oe). So in ZFC-FC curve, measured at 100 Oe field, we observed the magnetic moment of RGO-600 is higher than RGO. This can explain the difference in low field and high field magnetization at low temperatures in RGO and RGO-600. Magnetic behavior, especially of coercivity of RGO and RGO-600 are very similar, but we observe a different type of behavior in case of GO-600 sample. From Figure 4 and 7 we see that the magnetic moment increases drastically after annealing the GO at 600oC. This is in sharp contrast with reduction of moments due to chemical reduction. Magnetic coercivity of GO-600 decreases with increasing temperature (Figure 7) which is similar to a normal ferromagnetic material. As we described earlier, during annealing (GO at 600oC in Ar atmosphere) the epoxy groups migrates on the graphene surface (by acquiring thermal energy) and they self organize to form long chains of epoxy groups. At high temperatures these chains of epoxy groups can be collectively removed from GO sheets leading to unzipping of GO sheets into small fragments. In this process large numbers of zigzag edges at the boundary of the fragmented pieces are exposed. This will give rise to drastic increase in magnetization. Magnetic moments at the zigzag edges are strongly ferro-magnetically coupled as discussed earlier and so the coercivity increases with decreasing temperature like a normal ferromagnetic sample. From the magnetic hysteresis data it is clear that magnetic property of GO-600 differs from RGO and RGO-600. The magnetic property of GO-600 in mainly dominated by edges. Whereas on the other hand due to insufficient amount of epoxy groups in RGO, long chains of epoxy


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groups can not be formed by acquiring thermal energy and hence upon annealing the RGO unzipping of the sheets does not occur . Hence, RGO-600 remains almost same in size as RGO and concentration of zigzag edges does not increases after annealing unlike in GO-600. Hence, magnetic behavior of RGO-600 does not differ from RGO which is dominated by the defects like adatoms (-O-, -OH etc.) and atomic vacancies on its surface only. The coercivity of RGO-600 also decreases with decreasing temperature like in RGO due to the presence of wrinkles in the structure.33 But in case of GO-600, the magnetization is mainly dominated by the edges formed due to unzipping and hence behaves like normal ferromagnetic materials i.e. coercivity increases with decreasing temperature.

3 CONCLUSION

In conclusion, we have studied the magnetic property of chemically synthesized as prepared and annealed graphene oxide (GO) and reduced graphene oxide (RGO). We have observed significant increase in magnetization on annealing the GO whereas on annealing RGO the increase in magnetization is comparatively less. Significant increase in magnetic moment is due to the increase in the number of zigzag edges after annealing of GO indicates that the epoxy groups migrates over the GO surface at elevated temperatures and arranges themselves on the opposite sides of the hexagonal rings (which is the lowest energy configuration for this arrangement). The density of these epoxy groups play a big role in unzipping of the graphene resulting in greater number of zigzag edges. RGO samples do not have sufficient number of epoxy groups to unzip after annealing. The magnetic behavior of GO, RGO and RGO-600 is dominated by the adatoms and vacancies on its surface but the magnetic property of GO-600 is dominated by the zigzag edges. The coercivity of GO-600 behaves like those of a normal


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ferromagnetic material as these edges are not affected by wrinkles unlike GO, RGO and annealed RGO materials.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +91 33 2337 4637.

Notes The authors declare no competing financial interest.


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(40) Tuinstra, F.; Koening, J.L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 11261130. (41) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558–1565. (42) Krishnamoorthy, K.; Veerapandian, M.; Mohan, R.; Kim, S.-J. Investigation of Raman and Photoluminescence Studies of Reduced Graphene Oxide Sheets. Appl. Phys. A: Mater. Sci. Process. 2012, 106, 501–506.

(43) Cancado, L. G.; Pimenta, M. A.; Neves, B. R. A.; Dantas, M. S. S.; Jorio, A. Influence of the Atomic Structure on the Raman Spectra of Graphite Edges. Phys. Rev. Lett. 2004, 93, 247401-1–247401-4. (44) Casiraghi, C.; Hartschuh, A.; Qian, H.; Piscanec, S.; Georgi, C.; Fasoli, A.; Novoselov, K. S.; Basko, D. M.; Ferrari, A. C. Raman Spectroscopy of Graphene Edges. Nano Lett. 2009, 9, 1433–1441. (45) You, Y.; Ni, Z.; Yu, T.; Shen, Z. Edge Chirality Determination of Graphene by Raman Spectroscopy. Appl. Phys. Lett. 2008, 93, 163112-1–163112-3. (46) Bao, W.; Miao, F.; Chen, Z.; Zhang, H.; Jang, W.; Dames, C.; Lau, C. N. Controlled Ripple Texturing of Suspended Graphene and Ultrathin Graphite Membranes. Nat. Nanotechnol. 2009, 4, 562 –566.

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Figure Captions: Figure 1: (color online) Schematic diagram for (a) zigzag edge and armchair edge cut of graphene due to epoxy groups indicated by solid circles, (b) random arrangement of epoxy group (c) alignment of epoxy group on annealing (d) unzipping on reduction.

Figure 2: (color online) (a), (b) STEM-HAADF images of GO, RGO sample. (c), (d) Low magnification TEM and High-resolution TEM images of RGO-600.

Figure 3: (color online) Raman spectra of GO (a), GO-600 (b), RGO (c) and RGO-600 (d). Figure 4: (color online) Zero field cooled and field cooled (ZFC-FC) magnetization vs. temperature data of GO, RGO, RGO-600 and GO-600 sample in presence of 100 Oe magnetic field.

Figure 5: (color online) Magnetic Hysteresis curve of RGO at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.

Figure 6: (color online) Magnetic Hysteresis curve of RGO-600 at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.

Figure 7: (color online) Magnetic Hysteresis curve of GO-600 at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.


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Table of Content Title: Contrasting Magnetic Properties of Thermally and Chemically Reduced Graphene Oxide. Authors: Kousik Bagani, Mayukh K. Ray, Biswarup Satpati, Nihar R. Ray, Manas Sardar and Sangam Banerjee.


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Figure 1: (color online) Schematic diagram for (a) zigzag edge and armchair edge cut of graphene due to epoxy groups indicated by solid circles, (b) random arrangement of epoxy group (c) alignment of epoxy group on annealing (d) unzipping on reduction.


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Figure 2: (color online) (a), (b) STEM-HAADF images of GO, RGO sample. (c), (d) Low magnification TEM and High-resolution TEM images of RGO-600.


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Figure 3: (color online) Raman spectra of GO (a), GO-600 (b), RGO (c) and RGO-600 (d).


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Figure 4: (color online) Zero field cooled and field cooled (ZFC-FC) magnetization vs. temperature data of GO, RGO, RGO-600 and GO-600 sample in presence of 100 Oe magnetic field.


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Figure 5: (color online) Magnetic Hysteresis curve of RGO at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.


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Figure 6: (color online) Magnetic Hysteresis curve of RGO-600 at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.


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Figure 7: (color online) Magnetic Hysteresis curve of GO-600 at three different temperatures (10K, 100K and 300K). Inset: Zoom region near low field of the hysteresis curve.


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Contrasting Magnetic Properties of Thermally and Chemically

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