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Realization of Ambient Stable Room-Temperature Ferromagnetism by Low-Temperature Annealing of Graphene Oxide Nanoribbons Lin Fu, Yong Wang, Kaiyu Zhang, Weili Zhang, Jie Chen, Yu Deng, Youwei Du, and Nujiang Tang ACS Nano, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 27, 2019

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Realization of Ambient Stable Room-Temperature Ferromagnetism by Low-Temperature Annealing of Graphene Oxide Nanoribbons Lin Fu, Yong Wang, Kaiyu Zhang, Weili Zhang, Jie Chen, Yu Deng, Youwei Du, and Nujiang Tang* National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, PR China

ABSTRACT: The graphene oxide nanoribbons (GONRs) annealed at the low temperature of 400 oC (aGONRs-400) is developed as an excellent room-temperature (RT) ferromagnet. The saturated magnetization (Ms) of aGONRs-400 is high up to 0.39 emu/g at room temperature, and the RT ferromagnetism (FM) exhibits excellent ambient stability with Ms preserved for over half a year. The preferential distribution of the magnetic phenolic hydroxyl towards the edges, which contributes to the long-range ferromagnetic couplings, was confirmed by X-ray photoemission spectroscopy measurement and gradient annealing analysis. The approach of low-temperature annealing is proved to be efficient both to remove the prominent nonmagnetic epoxy groups on the basal plane of GONRs or transform them to magnetic hydroxyl groups, and to preserve the magnetic phenolic hydroxyl at the edges to realize the strong and ambient stable FM.

KEYWORDS:

room-temperature

ferromagnetism,

ambient

stability,

graphene

oxide

nanoribbons, low-temperature annealing 1 ACS Paragon Plus Environment

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Room-temperature (RT) ferromagnetism (FM) in carbon-based nanomaterials without transition metal or rare earth elements is of great significance in physics, chemistry, spintronics and materials science, for the tremendous advantages of carbon as widely available, bio-compatible and pure light-element organic materials with broad plasticity.1-7 Lowering the dimensionality of the carbon material is considered to be an effective way to offer more possibilities to manipulate its magnetism.7 Notably, graphene exhibits varieties of interesting physical phenomena, including tunnel magnetoresistance,8-10 quantum magneto-optical Faraday and Kerr effects,11 Rashba effect,12,13 the half-integer quantum Hall effect,5,14 and so on. Especially, due to the micron-sized spin diffusion length, graphene has great potential in spintronic devices.15 Nevertheless, the pristine graphene is intrinsically nonmagnetic for its symmetry π-conjugated network without unpaired electrons left.3,7,16 The lattice imperfections are reported to be the cause of the unpaired p electrons and, hence, the localized magnetic moments, while the distribution of which on the same sub-lattice leads to ferromagnetic couplings.17-19 Among the abundant methods to acquire FM in graphene materials, introducing the zigzag edges19 or periodic array of defects,20 doping with sp3 type foreign atoms or functionalization groups,21-23 and utilizing the proximity effect24,25 are frequently adopted. The prominent ferromagnetic graphene derivatives were mentioned in (reduced) graphene oxide, and the magnetic sources of which were mostly ascribed to the abundant configurations of defects induced during the oxidization or annealing process.26-28 However, the saturated magnetization (Ms) in such materials are usually weak and the mechanism of FM induced by the defects are

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controversial.16,18,29-31 Besides, the doping of different functional groups on the surface as the effective sp3-type candidate and the substitution of foreign atoms of p-block elements into the crystal lattice as the possible strategy are alternative ways to induce robust magnetism in graphene materials.23,32-35 Interestingly, among the doped graphene materials, the Ms of S-doped graphene has already reached ~5.5 emu g-1, accordingly, the doping density and substitution effect were identified as the cause of FM at low temperature.35 Moreover, the Ms in the low-temperature ferromagnetic state of vertical graphene grown from butter precursors has been reported to be even high up to ~8 emu g-1, and the dense atomic vacancies, hydrogenation, and edge states were contributed to the strong magnetism.36 Most of these materials, however, the Curie temperature (TC) of which are far below room temperature, and the ferromagnetic content just occupies a tiny part. Furthermore, the ambient stability of FM is quite significant to the real applications, however, the ferromagnetic features are usually difficult to maintain for a long time in the ambient conditions or rarely discussed. The edge states in graphene materials have been proved to be closely relevant to the ordered magnetism theoretically and experimentally, for the unpaired spins were left intrinsically in the zigzag edges.7,19,37 Moreover, the long-range distribution of the localized spins existing on the same lattice in graphene nanoribbons (GNRs) will lead to ferromagnetic couplings. Namely, it is an effective way to simultaneously induce localized spins and realize long-range ferromagnetic couplings in zigzag GNRs.19,22 Actually, the intrinsic RT FM in GNRs have been reported before.19,38,39 However, the bare zigzag edges are frequently unstable in ambient surroundings, and thus the induced FM could be passivated in a large degree.40 Therefore, realization of strong 3 ACS Paragon Plus Environment

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RT FM with ambient stability in graphene materials remains the key challenge, and is of great significance. Here, we report the realization of the intrinsic and ambient stable RT FM in graphene oxide nanoribbons (GONRs) by annealing of GONRs at 400 oC (aGONRs-400), and the high ferromagnetic magnetization reaches 0.39 emu/g. The low-temperature annealing was demonstrated to be responsible for the high magnetization as well as the stable edge configurations. Specifically, a large number of hydroxyl groups were reserved at the edges to saturate the unstable zigzag structure, which are ambient stability and afford the magnetic configurations in aGONRs-400 the high ambient stability.

RESULTS AND DISCUSSION

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Figure 1. Morphologies characterization of aGONRs-400. (a) TEM image of the isolated aGONRs-400. The dashed colorful lines indicate the typical bilayer structure of the sample. (b) Width distribution of aGONRs-400. The black line is the Lorentz function fitting. (c) AFM image of the typical aGONRs-400. The corresponding heights of the colorful lines regions are inserted.

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The double-walled carbon nanotubes (DWCNTs) were longitudinally unzipped to bilayer graphene oxide nanoribbons (the unzipping ratio is > 90%, Figure S1),39 and the as-prepared GONRs were then annealed at 400 oC. The morphologies of GONRs and annealed samples (Figure 1 and Figure S2) were measured by transmission electron microscopy (TEM) and atomic force microscopy (AFM). As shown in Figure 1a,c, the width and thickness of aGONRs-400 are of ~10.1 and 0.77-0.98 nm, respectively. Obviously, the dislocation of the upper and lower layer is observed in aGONRs-400 (Figure 1a), while the specific thickness further indicates the bilayer structure of the annealed GONRs. To make a clear sight of the morphologies, the corresponding width distribution of aGONRs-400 were calculated from more than 100 positions over 50 isolated samples (Figure 1b), and the average width (Wa) is ca. 9.9 nm. Therefore, the ratio of the edge carbon to the total C atoms (Re) is calculated as ca. ~4.35% (Table S1). The relative abundant edge C atoms would play a vital role in the generation of the localized unpaired spins at edges and thus the edge magnetism.

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Figure 2. Microstructure characterization of aGONRs-400. (a) The 2D peak in Raman spectrum. (b) The D and G peaks in Raman spectrum, and the calculated ratio of the two peak areas ID/IG = 0.86. The fine-scanned (c) C 1s and (d) O 1s spectrum. The four peaks of I1, I2, I3, and I4 represent carbonyl/carboxyl, epoxy/ether, hydroxyl and chemisorbed or intercalated adsorbed water molecules, respectively. The black dots are the experimentally measured data and the colorful solid lines are the fitted curves.

The microstructure takes a prominent role in the magnetic properties, and thus Raman 7 ACS Paragon Plus Environment

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spectra and X-ray photoemission spectroscopy (XPS) of aGONRs-400 were performed (Figure 2 and Table S1). As reported previously, the shape of the Raman spectra is closely related to the layer of graphene materials,41,42 which is adopted in the structural characterization of this work. Shown in Figure 2a is the second-order 2D peak in Raman spectrum of aGONRs-400, and the band is fitted well by four peaks 2D1A, 2D1B, 2D2A, and 2D2B with the full width at half maximum (FWHM) of 24 cm-1, which furtherly confirmed the bilayer structure of the annealed sample. Moreover, the crystallite size (La) is generally calculated by the integrated intensity ratio of D and G peaks (ID/IG) with the typical equation (Equation (1)):43,44 2.4 ∗ 10 ―10 ∗ 𝜆4 𝐿𝑎 = 𝐼𝐷/𝐼𝐺

(1)

where λ = 532 nm is the excitation laser wavelength. As shown in Figure 2b, the ID/IG is 0.86, and the calculated La is ca. 22.34 nm, which is much larger than Wa of ca. 9.9 nm. It implies that despite that some oxygen groups exist in aGONRs-400, its basal plane of ribbons is nearly perfect. Clearly, the perfect sp2 structure is large enough to realize the magnetic coupling between the two edges. To further explore the content of different functional groups in aGONRs-400, the fine-scanned C 1s and O 1s spectra were performed (Figure 2c,d). By strictly controlling each component peak fixed with the same FWHM and fitting the signals with the Voigt function (70% Gaussian and 30% Lorentzian characters) after subtracting a Shirley background, we tried to eliminate the subjective factors and make the deconvolution results accurate as far as possible. The C1s spectrum was thus carefully deconvoluted into five peaks, C-C sp2 (284.6 eV), C-OH (hydroxyl, 285.4 eV), C-O-C (epoxy/ether, 286.4 eV), C=O 8 ACS Paragon Plus Environment

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(carbonyl, 287.7 eV), and COOH (carboxyl, 288.8 eV).39,40,45,46 Besides, the four peaks of O 1s spectrum around 530.88, 531.93, 533.38, and 534.7 eV were assigned to the O doubly bound to aromatic C (C=O, carbonyl/carboxyl), the O singly bound to aliphatic C (C-O-C, epoxy/ether), the O singly bound to aromatic C (C-OH, hydroxyl) and the O in chemisorbed/intercalated adsorbed water molecules.23,46 The corresponding ratios of oxygen groups to the total C atoms show the quite similar content, while the slight deviation between the two spectra may be produced by the inevitable error in the process of deconvolution. Additionally, the singly bonded oxygen to carbon (hydroxyl and epoxy/ether groups) occupies much larger proportions than the doubly bonded forms (carbonyl and carboxyl groups). To confirm the oxygen groups’ species further, the Fourier transform infrared spectroscopy (FTIR) of aGONRs-400 was performed as a qualitative measurement, and the results show that the hydroxyl, epoxy, carbonyl, and carboxyl were all verified (Figure S3). Notably, the epoxy groups would begin to dramatically decrease at a quite low temperature (even at 200 oC) but may sustain at a higher temperature (600 oC, for instance), while the C-O-C components should be ascribed to the ether groups at high temperature (higher than 800 oC).45,46 Furthermore, when comparing the total content of the edge-bonded-only carbonyl and carboxyl groups to Re, one can conclude that over half of the edge carbon atoms were actually bonded by phenolic hydroxyl groups. Accordingly, the annealing process at 400 oC removed most of the epoxy groups or transformed them to hydroxyl groups, while some of the later ones may move to the edges accompanied by the mass loss of edge-bonded-only groups. Namely, the thermal stable oxygen groups like phenolic hydroxyl realize the preferential distribution towards the edges in the process of basal-plane-type groups 9 ACS Paragon Plus Environment

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like epoxy losing or transforming. With the decrease of oxygen groups on the basal plane of GONRs, the integrity of the basal plane would be improved, which may be the reason that annealed GONRs have a nearly perfect basal plane as mentioned above.

Figure 3. Magnetism of aGONRs-400. (a) ZFC and FC M-T curves measured from 2 to 400 K under the applied field H = 0.1 T. M-H curves measured at (b) 2 and (c) 300 K. Top left inset of (b): M-H curves for paramagnetic (Mpara) and ferromagnetic (Mferro) components of the total magnetization (Mtotal). Lower right inset of (b): magnification of M-H curve at 2 K from -0.2 to 0.2 T. (d) Magnification of M-H curve at 300 K from -0.35 to 0.35 T, Mo was marked with a 10 ACS Paragon Plus Environment

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black arrow.

As well-known that the edge states in GNRs are of vital significance to induce the localized magnetism,19,37 while the preferential distribution of oxygen functionalization groups at the edges in aGONRs-400 may contribute to the preservation of edge states,47 and thus result in the interesting magnetic properties. Therefore, the magnetic properties of aGONRs-400 were measured by superconducting quantum interference device (SQUID) magnetometer a month later after it was synthesized. Zero-field-cooled (ZFC) and field-cooled (FC) M-T curves measured at 2-400 K were shown in Figure 3a, in which the observable divergence suggests the ordered magnetism appeared in aGONRs-400. Moreover, the large magnetization far from zero even at 400 K may be a feature of high TC over the room temperature, while this high-temperature stable magnetism of aGONRs-400 makes it more suitable for the real application in spintronic devices. Figure 3b shows the M-H curve measured at 2 K of aGONRs-400. It is found that the Ms is high up to 5.36 emu/g. Furthermore, the magnification of the M-H curve measured at 2 K from -0.2 to 0.2 T shows the typical coercive field (Hc) and remnant magnetization (Mr) as 368 Oe and 0.19 emu/g, a solid evidence of the FM. Namely, both paramagnetism (PM) and FM coexist in aGONRs-400 at 2 K. Correspondingly, the magnetization at 2 K can be expressed as Mtotal = Mpara + Mferro (inset of Figure 3b), in which the Ms of paramagnetic signal is ca. 3.39 emu/g, while the ferromagnetic component is high up to ca. 1.97 emu/g. Interestingly, different from the case at 2 K, the rapid saturation of the magnetization with the increase of the applied field measured at 11 ACS Paragon Plus Environment

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300 K indicates a pure ferromagnetic feature (Figure 3c) with TC over room temperature. More importantly, the Ms is high up to 0.39 emu/g, which is the strongest pure RT FM reported in graphene materials to the best of our knowledge (Table S2). Besides, as can be seen in the hysteresis loop measured at 300 K from -0.3 to 0.3 T (Figure 3d), the Hc and Mr respectively are 218 Oe, and 0.1 emu/g, a typical ferromagnetic behavior. Additionally, the specific magnetization value at the overlapping point (Mo, H = 0.3 T) is about 0.34 emu/g, which is quite close to the Ms, confirming the pure ferromagnetic phase of aGONRs-400 at 300 K when the measuring factors are taking into consideration. Considering the facts that (i) the inter-edge couplings would drive an FM state when the GNRs is wider than 8 nm,19 and (ii) the Wa of aGONRs-400 is ca. 9.9 nm, reasonably, the inter-edge couplings in our sample may contribute to the observed FM. Furthermore, to exclude the effect of the magnetic impurities on the ferromagnetic features, the inductively coupled plasma (ICP) measurements were employed. The result showed that the specific content of the impurities can be neglected with Fe < 7 ppm, Mg < 13 ppm, and the other elements lower than the detected limit (Table S3). Namely, the RT FM discussed here is the intrinsic feature of the sample.

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Figure 4. Ambient stability of the microstructure and magnetism in aGONRs-400. (a) The ratios of oxygen and groups to C atoms measured immediately and 6 months later after the sample was synthesized. Inset is the specific relative contents change (%) of the main magnetic oxygen groups (OH) before and after exposure for 6 months. (b) The RT FM evolution after exposure for 0, 1, and 6 months.

The ambient stability is important for the real application of GONRs, and which was measured to further evaluate the microstructure and magnetism evolution (Figure 4 and Figures S4-S7). Both GONRs and aGONRs-400 were exposed under the ambient conditions for a month and even over half of a year. Figure 4a shows the microstructure evolution before and after exposure for over 6 months of aGONRs-400. Obviously, only a slight increase of oxygen and the main magnetic OH groups (inset of Figure 4a) can be observed in aGONRs-400, while a slump of both the oxygen and the main magnetic group (COOH) occurred in GONRs (Figure S4). Reasonably, the evolution of the microstructure may be closely relevant to the magnetic 13 ACS Paragon Plus Environment

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properties. Shown in Figure 4b is the FM evolution at 300 K of aGONRs-400, in which the Ms intensities measured after exposure for 1 and 6 months mostly maintain comparing with that of the sample without ambient exposure. By contrast, a normal phenomenon occurred in GONRs that its Ms suffered a significant decrease (especially re-measured after exposure for 6 months, Figure S5), which is similar to the cases in other carbon materials.48 A similar trend occurred at 2 K in both of the two samples (Figure S6), while the relative intensities of 2 and 300 K Ms to that of the same samples without ambient exposure (Figure S7) indicate the phenomenon more clearly. Namely, the magnetism in aGONRs-400 shows much higher ambient stability than in GONRs. One can propose that the instability of both the edges and oxygen groups on the basal plane are the cause of magnetism suppression in GONRs, while the preferential distribution of the ambient stable hydroxyl groups at the edges of aGONRs-400 is responsible for its enhanced ambient stable magnetism. To further show the dependence between the phenolic hydroxyl groups and ambient stable magnetism, the as-prepared GONRs were annealed at the higher temperatures. Reasonably, both the microstructure and magnetism in GONRs annealed at 800 and 1000 oC (aGONRs-800 and aGONRs-1000, Figures S8-S10, Table S4 and S5) also show similar ambient stability as aGONRs-400. Besides, we notice that the 2 K M-H curves in aGONRs-800 and aGONRs-1000 show the observable unsaturated features, which may be a typical antiferromagnetic behavior like the previously reported high-temperature-annealing GONRs.39 As known that the antiferromagnetism in GNRs originates from the interlayer couplings, while the couplings can be significantly suppressed by the widely distributed oxygen groups on the basal plane in aGONRs-400, which may be the reason why no observable 14 ACS Paragon Plus Environment

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antiferromagnetic behavior appeared in the low-temperature annealed sample.

Figure 5. Annealing temperature-dependence RT FM of GONRs. (a) RT Ms of GONRs, aGONRs-300, 400, 500, 600, 800 and 1000 (GONRs, aG-300, aG-400, aG-500, aG-600, aG-800, aG-1000). (b) The comparison of hydroxyl groups, magnetic groups (OH+COOH) and the oxygen groups which can bond only at the edges (COOH+C=O), and the dashed line marked the Re in different samples. (c) Schematic illustrations (top view of single layer) of the microstructure configurations transition in GONRs (left), the low-temperature (LT) annealed GONRs (middle), and the high-temperature (HT) annealed GONRs (right). The gray and white balls are C and H atoms, while the red, green, dark blue and light blue balls are O atoms in the epoxy/ether, carbonyl, carboxyl, and hydroxyl groups.

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To clarify the dependence of the RT FM to the annealing temperature, the microstructure transition and Ms evolution in thermal gradient annealed samples of GONRs, aGONRs-400, aGONRs-800, aGONRs-1000, and the GONRs annealed at 300, 500, 600 oC (aGONRs-300, aGONRs-500, and aGONRs-600) are shown in Figure 5. The pure ferromagnetic features are found in all the samples at 300 K (Figure 5a), as mentioned above in the situation of aGONRs-400. Besides, aGONRs annealed at low-temperature (300-600 oC) present stronger FM than GONRs, while a weak decrease appears in the high-temperature (800-1000 oC) annealing cases. Actually, the oxygen functional groups related microstructures are responsible for the magnetic behaviors of oxidized graphene materials,47,49 and all the oxygen species and related microstructure transition in different-temperature-annealing samples are thus given (Figures S11-S14, Table S4 and S6). It is known that among the oxygen groups chemisorbed at zigzag edges of the graphene sheet, (i) the C-O-C and C=O are nonmagnetic39 despite which can be magnetic in other thin layers;50-54 however, (ii) the COOH and OH are magnetic.39 The most obvious change in these oxygen groups is the extremely reducing of nonmagnetic C-O-C components in low-temperature annealed GONRs followed by a further decrease in high-temperature annealed samples compared with GONRs. The other nonmagnetic component of C=O shows a similar tendency. For the magnetic components, the ratio of COOH decreases with the increase of the annealing temperature, while the highest ratio of OH groups appears in aGONRs-400. To further analyze the effect of the magnetic groups in RT FM, the ratio of OH groups together with the sum of OH and COOH groups are drawn in Figure 5b. Interestingly, the total ratios of magnetic oxygen 16 ACS Paragon Plus Environment

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groups are inconsistent with the Ms in different samples, on the contrary, OH groups just follow the same trend. On the one hand, the massive nonmagnetic groups suppress the ferromagnetic couplings in GONRs and the samples annealed at low temperatures. On the other hand, the low magnetic efficiency of the unstable COOH at the edges results in their weak contribution to the RT FM. Besides, as discussed above, the defects-induced FM may just occupy a tiny portion of the RT Ms. Therefore, one can propose that the OH groups play a vital role in the strong FM in the samples annealed at low temperatures (especially for aGONRs-400). Additionally, the edge-bonded-only groups and Re are also shown in Figure 5b, and one can find that when the annealing temperature reaches 400 oC, the ratio of the sum of COOH and C=O to C atoms is much lower than Re. Considering the higher activity of the edge C atoms and the stronger thermal stability of the OH groups at the edges than those on the basal plane, the OH groups are thus confirmed to be preferential to bond at the edges as phenolic hydroxyl.40 Clearly, the FTIR results confirm further the substantive presence of phenolic hydroxyl in aGONRs-400 (Figure S3). As reported that the oxidative cutting processes of graphene materials were mostly along the zigzag direction.55,56 Correspondingly, the schematic illustrations for the microstructure configurations of the samples are shown in Figure 5c, and the preferential distribution of hydroxyl groups towards the edges in the samples annealed at low temperatures contribute to the ambient stable and strong RT FM. It is reasonable that the FM Ms of GONRs would be dependent on both the content of the magnetic phenolic hydroxyl at the edges and the concentration of defects such as oxygen-doping on the basal planes. Note that the inevitable defects induced by oxidative unzipping of CNTs 17 ACS Paragon Plus Environment

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usually significantly affect the intrinsic properties of the samples,57 and thus one cannot exclude the possibility that the defects in the basal planes of ribbons can also contribute to the magnetism of annealed GONRs. It is found that the La becomes larger with the increase of the annealing temperature (Table S6). It indicates the concentration of defects decreases, which may attribute to the deoxidation of GONRs after annealing (Figure S12). Furthermore, based on the hole-doping type of the oxygen groups, the downward shift of G peak implies further the decrease in the doping level.58 Theoretically, the inter-edge coupling diagrams of GNRs with the width ~10 nm would be FM in the low doping condition, and change to be PM eventually in the high doping situation.19,59 Namely, compared with the case of GONRs, the much lower oxygen-doping level on the basal plane of annealed GONRs motivates the ferromagnetic coupling between the two edges. Consequently, the FM Ms of annealed GONRs shows a significant increase after the annealing. However, the variation in the content of phenolic hydroxyl makes the Ms of annealed GONRs increase first and then decrease with the increase of the annealing temperature (Figure 5a,b). Furthermore, as discussed above, despite that some defects inevitably exist in the basal planes of our annealed GNRs, these basal planes are nearly intrinsic. In other words, the defects may contribute to magnetism, but the contribution is tiny. Besides, the decrease in defects in the basal planes of our annealed GNRs with increasing annealing temperature would reduce this contribution further. Therefore, one can make the reasonable assumption that (i) the phenolic hydroxyl saturated zigzag edges are the dominant magnetic sources,40,47 and (ii) the decreasing of the oxygen-doping level in annealed GONRs comparing with GONRs motivates the ferromagnetic coupling between the two edges. 18 ACS Paragon Plus Environment

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Additionally, our detailed studies showed that the magnetic properties are closely relevant to the reaction conditions in the unzipping process. Specifically, when the unzipping conditions are too weak or violent, low unzipping ratio or fusion of the GONRs edges may occur in as-prepared samples (Figure S15). Consequently, much weaker magnetism would be obtained, while the ferromagnetic features even disappeared (Figure S16). Therefore, the excellent magnetic properties would be available by strictly controlling the reaction conditions.

CONCLUSIONS In summary, aGONRs-400, an excellent graphene RT ferromagnet, is developed via annealing of GONRs at 400 oC. By employing the gradient annealing analysis and XPS measurements, the microstructure of aGONRs-400 and the preferential distribution of the magnetic phenolic hydroxyl towards the edges and thus achieving the long-range ferromagnetic couplings were confirmed. In particular, we realize the strongest RT pure FM with the Ms high up to 0.39 emu/g, and it exhibits ambient stability with the Ms mainly maintained for more than 6 months. The gentle approach of low-temperature annealing is proved to contribute not only to the high Ms but also to the excellent ambient stability. The proper optimization of the microstructure and physical comprehension offer insight into the design of strong and ambient stable organic ferromagnets.

METHODS Synthesis of GONRs. GONRs were synthesized by longitudinal unzipping of DWCNTs 19 ACS Paragon Plus Environment

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(Nanjing XFNANO Materials Tech Co., Ltd., 2-4 nm in diameter and 0.5-30 m in length). The pristine DWCNTs were first stirred in hydrochloric acid to dissolve the transition metals and then put into oxalic acid solution for intercalation before unzipping. The oxalic acid-intercalated DWCNTs were then unzipped to bilayer GONRs with a typical procedure.39 Synthesis of aGONRs-400. The GONRs power was put into a quartz boat, then flushed with argon for 1 h in a quartz tube furnace to discharge the air. Then the sample was annealed at 400 oC for 1 h in argon with a gentle flow speed. After the system cooled to room temperature, aGONRs-400 power was obtained. Sample characterization. The morphologies of our samples were observed by TEM (Model JEOL-2010, Japan) and AFM (SPI-3800N, Japan). Raman spectra were obtained by confocal Raman microscope (LabRAM Aramis, Japan) using a laser excitation of 532 nm. XPS measurements were performed on PHI-5000 VersaProbe using Al Kα radiation operated in a residual vacuum of 5  10-9 Torr. The magnetic properties of the sample were measured by SQUID magnetometer with the sensitivity less than 10-8 emu (MPMS-XL, USA). The 3d impurity elements were examined by ICP spectrometry (Jarrell-Ash, USA) with the detection limit lower than 0.01 ppm.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Additional details on the morphologies, microstructures, magnetism and ICP measurements 20 ACS Paragon Plus Environment

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of GONRs, aGONRs-300, aGONRs-400, aGONRs-500, aGONRs-600, aGONRs-800 and aGONRs-1000, as well as GONRs-1 and GONRs-2 unzipped in different reaction conditions, and the reported RT FM in graphene materials (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Lin Fu: 0000-0002-0469-6645 Jie Chen: 0000-0002-7183-0904 Nujiang Tang: 0000-0003-4541-2183 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the State Key Program for Basic Research of China (Grant No. 2017YFA0206304), NSFC (Grant No. 51572122), and Natural Science Foundation of Jiangsu Province (Grant No. BK20151382), China. A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS.

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ToC figure

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Table of Contents. The low-temperature annealing is used to realize the preferential distribution of phenolic hydroxyl groups towards the edges in aGONRs-400 for saturating the ambient unstable bare edges. This well-designed organic ferromagnet shows strong room-temperature ferromagnetism with excellent ambient stability.

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