Structure and Magnetic Properties of Pristine and Fe-Doped Micro

Article pubs.acs.org/JPCC

Structure and Magnetic Properties of Pristine and Fe-Doped Microand Nanographenes A. M. Panich,*,† A. I. Shames,† M. I. Tsindlekht,‡ V. Yu. Osipov,*,§ M. Patel,∥ K. Savaram,∥ and H. He∥ †

Physics Department, Ben-Gurion University of the Negev, 8410501 Be’er Sheva, Israel Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel § Ioffe Physical-Technical Institute, St. Petersburg 194021, Russia ∥ Chemistry Department, Rutgers University, Newark, New Jersey 07102, United States ‡

ABSTRACT: We report on magnetic susceptibility, NMR, and EPR measurements of pristine and Fe-doped micro- and nanosized graphenes (LGr and NGr), prepared by a unique microwave enabled technique from graphite particles. Significant orbital diamagnetism in the studied compounds (∼70% of that of bulk graphite) is revealed. At T < 30 K, a weak paramagnetism due to edge π-electronic spin states is observed. Reduction on the lateral size of the graphene sheets results in the suppression of orbital diamagnetism and strengthening of the paramagnetic contribution due to an increased number of open edges in NGr. Significant acceleration of 13C nuclear spin−lattice relaxation under iron doping of both LGr and NGr samples is attributed to the interaction of nuclear spins with paramagnetic Fe ions and indicates that the latter are anchored to the graphene edges. The amount of Fe ions attached to the edges of NGr-Fe is ∼6.6 times higher than that of LGr-Fe. This finding reveals the noticeable capability of nanographene to fix Fe ions on its periphery terminated by various oxygen-containing groups and atomic hydrogen. Several schemes for such fixation are proposed.

1. INTRODUCTION The discovery of graphene led to the exploration of various techniques for its mass production. The main attention in the literature has been given to the Hummers method and its modifications, where graphite micropowder is treated by solutions of concentrated H2SO4, NaNO2, and KMnO4 as a primary oxidizing agent to fabricate graphene oxide (GO) flakes. The GO obtained is then reduced to so-called reduced graphene oxide (r-GO) with a lower amount of oxygencontaining functional groups to recover the properties of intrinsic graphene. Graphene sheets obtained by this method vary in size and exhibit defects (the size of a coherent defectfree region normally is 100 K and also at T < 40 K because the adsorbed paramagnetic oxygen goes into a nonmagnetic state below ∼50 K forming antiferromagnetically coupled O2−O2 dimers with zero spin. Therefore, both intrinsic dia- and paramagnetic contributions can be well separated and documented as shown in Table 1. The positive slope above 100 K for NGr, LGr, and LGr-Fe χ(T) curves in Figure 2, that is, decrease in orbital diamagnetism on heating, is caused by the shift of the Fermi level and Fermi step smearing with increasing temperature.7 Curie constants C, Weiss temperatures Θ, diamagnetic contributions to magnetic susceptibility χdia, density of prevailing paramagnetic agents NS, and 13C spin−lattice relaxation times T1 of graphene samples are given in Table 1. The polynomial coefficients of the temperature dependence of the orbital susceptibility received by simulation of the experimental data are given as well. Powder spatially averaged orbital diamagnetism is maximal for micrographene LGr (1/3·χorb(300 K) = −4.4 × 10−6 emu/g) and its iron-modified derivative LGr-Fe (1/3·χorb(300 K) = −5.5 × 10−6 emu/g), which is around 62−77% of that of powder microcrystalline graphite. The slight increase of orbital diamagnetism of LGr on iron doping might be caused by a “reverse” shift of the Fermi level (due to compensation of holes of intrinsic acceptor centers released by iron electrons) close to the Dirac point, in which, according to the theory, diamagnetic singularity and maximal diamagnetism are reached. (We note that for an ideal infinite graphene sheet with Fermi energy level at 0 eV relative to the Dirac point, the orbital diamagnetism at T → 0 tends to infinity as 1/T). In the LGr sample, the low temperature Curie−Weiss term is caused by both intrinsic π-electron spin states on zigzag-type graphene edges and remaining isolated triplet oxygen molecules physisorbed on the basal planes of graphene, which do not create spin-zero dimers below 40 K. Suggesting the first group of spins to be S = 1/2 and the second one to be S = 1, the total density of both types of paramagnetic centers is roughly estimated as ∼7 × 1018 spin/g. Similar considerations are valid for the NGr sample. However, the difference between spin densities determined by low T SQUID and RT EPR (Table 1) most probably originates from the stronger effect of oxygen to NGr edge-spins at RT, causing strong broadening and, thus, nonobservability of some edgespins.17 In the iron-doped LGr-Fe sample, the Curie−Weiss term is mainly caused by paramagnetic iron ions, presumably both Fe2+ with S = 2 and Fe3+ with S = 5/2 ions, whose total

, where μB is the Bohr magneton,

kB is the Bolzmann constant, S is the total spin of paramagnetic center, g is the Lande factor, and χo is the temperatureindependent term in magnetic susceptibility summing all nonCurie components in the low-temperature range. Temperature dependencies of the magnetic susceptibility χ of the samples under study are shown in Figure 2 and reveal

Figure 2. Temperature dependencies of magnetic susceptibility of the powder graphene samples LGr, NGr, LGr-Fe, and NGr-Fe. Simulations of the experimental data by analytical function a + bT + C/(T − Θ), where the two first terms approximately describe temperature dependence of the diamagnetic contribution to the magnetic susceptibility, shown by blue lines. Parameters a, b, C, and Θ are given in Table 1.

two well-featured contributions. In the LGr, NGr, and LGr-Fe samples, the diamagnetic contribution characteristic of graphite-like carbon systems dominates in the wide range from RT to ∼30−40 K. At lower temperatures (T < 40 K), Curie-like paramagnetic contribution is dominating. For the NGr-Fe sample, the latter is prevailing in the whole temperature range from 4.5 to 300 K. χ(T) dependencies of LGr, NGr, and LGrFe samples exhibit some characteristic “humps” in the range of 45−90 K caused by physical adsorption of residual molecular oxygen on the graphene surface at T < 100 K and its transition into antiferromagnetic state at T < 60 K. (Although the measurements were carried out in the low pressure (∼10 kPa) helium atmosphere persisting in the SQUID chamber, some residual traces of atmospheric air are still present on the ≤1 kPa level depending on the actual leakage of vacuum seals and duration of measurements.) The above-mentioned residual oxygen starts to physisorb progressively on the graphene

Table 1. Curie Constant C, Weiss Temperature Θ, Diamagnetic Contribution to Magnetic Susceptibility χdia, Density of Prevailing Paramagnetic Agents NS, and 13C Spin−Lattice Relaxation Time Tn1 of Graphene Samples NS, spin/g EPRd

SQUID label LGr LGrFe NGr NGrFe a

material large graphene large graphene with iron nanographene nanographene with iron

C, emu × K/g

Θ, K

χdia = χcore + 1/3 × χorb(T), emu/ga

6.10 × 10−6 5.89 × 10−5

−1.5 −3.1

−6.32 × 10−6 + 6.60 × 10−9 T −8.12 × 10−6 + 8.10 × 10−9 T

1.18 × 1019b

1.30 × 10−5 3.89 × 10−4

−1.5 −1.45

−4.10 × 10−6 + 3.15 × 10−9 T −2.70 × 10−6

7.81 × 1019c

iron ions

spins S = 1/2 and triplet oxygen

spins S = 1/2

Tn1, s

∼7 × 1018

3.7 × 1018 1.3 × 1018

90 ± 16 19 ± 3

∼1.4 × 1019

1.1 × 1018

49 ± 7 4.9 ± 1.4

χcore ≈ −0.35 × 10−6 emu/g. bEstimated error ±11%. cEstimated error ±3.5%. dMeasured at T = 295 K, estimated error ±15%. D

DOI: 10.1021/acs.jpcc.5b11732 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C concentration is estimated as ∼1.2 × 1019 spin/g, which corresponds to 240 ppm of Fe in the material. Figure 3 shows magnetization curves (field dependencies of magnetization) of the localized spins obtained at T = 4.5 K for

concentration corresponds to approximately one iron ion for ∼6500 carbon atoms. The experimental data for the sample NGr-Fe (Figure 3) are also well described by the Brillouin function for the magnetization of the system of noninteracting isolated spins using the following triples of the adjusting parameters: g = 2.3, S = 2, NS = 5.2 × 1019 spin/g or g = 2.0, S = 5/2, NS = 4.75 × 1019 spin/g. Because of some uncertainty in the g-factor for Fe ion (2.0−2.3), it is not possible to determine the actual spin state (S = 2 and S = 5/2), indicating a possible simultaneous presence of both Fe2+ and Fe3+ ions in NGr-Fe. Nevertheless, the average value of the iron ion concentration of ∼5 × 1019 spin/g obtained by this method corresponds well to the value of 7.81 × 1019 spin/g (Table 1) obtained from the Curie constant. The obtained concentration corresponds to ∼1 iron ion for ∼1000 carbon atoms (equivalent to ∼1000 at. ppm), which is a very high value for doped nanocarbon systems. Thus, we can conclude that the use of magnetometric methods allows identification of Fe ions in LGr-Fe and NGr-Fe compounds. As a result of the substantial reduction of the lateral graphene size, from 0.5−3.0 μm in micrographene to ∼13 nm in nanographene, the RT orbital diamagnetism in nanographene NGr is suppressed as compared to LGr down to 1/3·χorb(300 K) = −3.1 × 10−6 emu/g, and the density of the localized edge spins 1/2 reveals an approximately 2-fold increase up to ∼1.4 × 1019 spin/g. Iron doping of NGr suppresses the orbital diamagnetism down to the temperature-independent value of 1/3·χorb(300 K) = −2.7 × 10−6 emu/g. Herewith the concentration of the Fe ions that attached to the nanographene edges is ∼7.8 × 1019 spin/g, that is, 6.6 times larger than that of LGr-Fe. The Curie constant in NGr-Fe becomes 30 times larger than that of undoped NGr sample, corresponding to an iron concentration of ∼1600 ppm. Thus, substantial reduction of the lateral size of the graphene microflakes results in a more than 6-fold increase in their functional capacity to the Fe ions. This property is most likely related to the at least 6-fold increase in the total length of the edges in the NGr system as compared to LGr, which are functionalized by appropriate short chains of neighboring carbonyl groups of >CO type, probably alternating on the graphene edge with hydroxyl groups >C−OH, while the number of other edge sites, functionalized by atomic hydrogen, and −COOH groups, is not so important for irreversible fixation of iron ions. Suppression of the orbital diamagnetism under iron doping of NGr is presumably caused by a Fermi level shift deeper into one of the bands, that is, further from the Dirac (i.e., diamagnetic singularity) point. A large value of the orbital diamagnetism obtained in micrographene (LGr) along with the positive slope of the temperature dependence of χ of (7−8) × 10−9 emu/(g × K) reveal the superior quality of the graphene under study, which is often unattainable by using other preparation methods. Herewith a number of other nanocarbon compounds (multiwall nanotubes, astralenes) reveal even higher values of orbital diamagnetism. Let us discuss in more detail the orbital diamagnetic component in nanographene NGr, whose temperature dependence is well visible and has a positive slope of 3.15 × 10−9 emu/ (g × K). Experimental data for sample NGr especially recalculated according to the formula χcalc(T) = 3·(χ(T) − χcore − χCW) are plotted in Figure 4. This dependence can be analyzed in the frameworks of Kotosonov’s band model of disordered quasi-two-dimensional graphite-like materials:8,9

Figure 3. Magnetization of localized spins M − χoB0 in iron-doped samples (NGr-Fe, ○; and LGr-Fe, △) versus magnetic field, obtained after subtracting the contribution of the diamagnetic term, at T = 4.5 K. Red curves show simulation by Brillouin formula with the fitting parameters g = 2.1, S = 2, NS = 7.6 × 1018 spin/g for LGr-Fe and g = 2.3, S = 2, NS = 5.2 × 1019 spin/g for NGr-Fe. (Alternative fitting for NGr-Fe may be done with the close parameters g = 2.0, S = 5/2, NS = 4.75 × 1019 spin/g.) The spin values obtained prove the dominant presence of iron ions but not the S = 1/2 or S = 1 spins.

Fe-doped LGr and NGr samples, after deducing from the total magnetization a linear magnetic field B0 contribution associated with the temperature-independent or weakly temperaturedependent components, that is, M − χoB0, where χo = χcore + 1 /3·χorb(4.5 K) + χPauli. As known, the magnetization of localized spins at low temperatures, determined by the average projection of the individual spins to the direction of the magnetic field B0, is described by the Brillouin expression: M = ⟨Mz⟩ = NSgSμB BS(x)

(2)

where x ≡ gSμBH/kBT. Here, S is the total spin of the paramagnetic center, prevailing in the system and responsible for its paramagnetism, NS is the concentration of the isolated paramagnetic centers, and BS(x) is the Brillouin function: BS(x) =

(2S + 1)x 1 2S + 1 x coth coth ‐ 2S 2S 2S 2S

(3)

Equations 2 and 3 suggest Boltzmann statistics for the spin ensemble regardless of their nature, the discrete energy spectrum of the magnetic moment in a magnetic field caused by the discrete projections of the spin ms on the direction of magnetic field B0, and the ensemble consisting of identical noninteracting magnetic moments. Paramagnetism of the localized spins that nonlinearly depends on the magnetic field (so-called “saturating nonlinearity”) is most clearly identified at very low temperatures, where the field dependence is specific for the different spins. The field dependence of magnetization for the LGr-Fe sample (Figure 3) is well described by the Brillouin function for a system of noninteracting spins using two independent variables, the spin S and the concentration of spins NS as adjustable parameters. The g-factor was assumed to be 2.1. For LGr-Fe we obtained S = 2 and NS = 7.6 × 1018 spin/g. The obtained value of S = 2 perfectly coincides with the theoretical value of the spin of the iron ion Fe2+ (S = 2), and the Fe E


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The Journal of Physical Chemistry C

uncoupled electron spins of the ion, which significantly accelerates nuclear spin−lattice relaxation. Herewith if magnetic inclusions are contained in a material as a separate phase, this effect is negligible. 13 C spectra of the graphene samples under study comprise broad asymmetric lines (Figure 5), characteristic of aromatic

Figure 4. Temperature dependence of in-plane orbital diamagnetic susceptibility summed with a priori unknown Pauli susceptibility of powder NGr sample after subtraction of low temperature paramagnetic Curie contribution C/(T − Θ) and temperatureindependent diamagnetic core contribution of carbon atoms χcore (with data conversion from spatially random powder orientations of crystallites to the planar ones for magnetic field perpendicular to the graphene sheets). Red line shows simulation of the experimental data by Kotosonov’s expression (eq 4); parameters ΔT, EF, and χPauli are given in the text. A hump in the region of 45−95 K comes from residual paramagnetic oxygen physisorbed on nanographene sheets.

Figure 5. Typical 13C NMR spectrum of graphene.

carbon atoms with sp2 hybridization.11,12 Measurements of the nuclear spin−lattice relaxation time T1 of graphene carbons show that the magnetization recovery in all samples is well described by a stretched exponential function M(t) = M∞{1 − exp[−(t/T1)α]}, where M∞ is the equilibrium magnetization and α is the stretched exponential parameter. The obtained relaxation parameters are given in Table 1. The spin−lattice relaxation times T1(13C) are 90 ± 16 and 49 ± 7 s for large graphene and nanographene samples, respectively. Herewith T1(13C) is significantly reduced in graphene samples doped by Fe ions, showing 19.5 ± 3 s for LGr-Fe and 4.9 ± 1.4 s for NGr-Fe, respectively (Table 1). In conductive materials, there are two main contributions to the nuclear spin−lattice relaxation caused by the interactions of nuclear spins (i) with the spins of conduction electrons and (ii) with paramagnetic defects. The former is described by the

χorb [emu/g] = − 1.3625 × 10−3γo2 ×

sech 2(E F /2kB(T + ΔT )) (T + ΔT )

(4)

Here, γo ≈ 3 eV is the 2D band parameter of graphene sheet, EF is the average Fermi energy measured from the contact point of π and π*-bands, ΔT is the additional term for nonthermal electron scattering on the boundaries and structural defects of graphene nanoparticles or, in other words, the “smearing” of the density of states of the ensemble of nanographene flakes in the vicinity of the Fermi level. This can occur for various reasons, including small fluctuation of the electrostatic potential caused by the nonuniform distribution of mobile carrier charges over the ensemble of particles. The phenomenological expression 4 is applicable for description of diamagnetism in different disordered graphite-like materials, from multiwall nanotubes to carbon onions. The values ΔT and EF were defined as fitting parameters at simulation of the recalculated and replotted experimental curve χorb(T) + 3χPauli = 3·(χ(T) − χcore − χCW) with eq 4. Here, ΔT is ∼890 K (or ∼80 meV), and the Fermi level at zero temperature is ∼33 meV. χPauli obtained as a remaining third fitting parameter, which causes only the vertical shift of the analytical function χorb(T) in the upper direction, is 0.63 × 10−6 emu/g and coincides well with the values obtained earlier for nanographites of onion-like type.10 The above Fermi energy value seems to be reasonable on suggestion of self-doping of impurity-free graphene that is due to the intrinsic acceptor defects as it occurs in a bulk graphite crystal. 3.2. 13C and 1H NMR. The question arises whether the aforementioned iron ions are bound to the graphene sheets or are contained in the compound as a separate phase. This problem was solved by our 13C NMR spin−lattice relaxation measurements. The matter is that paramagnetic ions, being bound to the graphene sheets, create an additional relaxation channel due to the interaction of carbon nuclear spins with

Korringa relation

1 TT 1

=

4πkBγn2 ℏγe2

K 2 . Here, kB and ℏ are the

Boltzmann and Planck constants, γn and γe are the nuclear and 8π electron gyromagnetic values, and K = 3 μB2 N (E F)|Ψ(0)|2 is the Knight shift that occurs due to contact interaction between the nuclei and conduction electrons and is proportional to the electronic density of states N(EF) at the Fermi level and to the electron probability density at the nuclear position averaged over the Fermi surface ⟨|Ψk(0)|2⟩EF. The contact (or δ-function) interaction restricts this effect to s-electrons. Because of the πtype of the conduction electrons in graphene sheets having a node at the nucleus site (⟨|Ψk(0)|2⟩EF = 0), there should be no direct isotropic 13C Knight shift and corresponding contribution to the relaxation in graphene and graphite. Therefore, the measurement of the 13C spin−lattice relaxation time in the LGr sample with relatively low density of the localized paramagnetic defects (mainly unpaired electron spins on the edges of graphene particle), NS = 3.7 × 1018 spin/g, reveals a rather long T1 = 90 s (Table 1), which is comparable with T1 = 110 s determined in polycrystalline graphite.1,11,12 It means that the contribution of the conduction electrons to the spin−lattice relaxation in the studied semimetallic graphene is as small as F


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The Journal of Physical Chemistry C that in graphite. However, the LGr-Fe sample under study exhibits a much shorter (as compared to LGr) spin−lattice relaxation time (T1 = 19 s), despite that the amounts of the intrinsic localized paramagnetic defects in these two samples are of the same order of magnitude and even somewhat smaller in LGr-Fe as compared to LGr (according to the EPR data presented in the next section; see Table 1). The only reason for the obtained T1’s reduction is the interaction of the 13C nuclear spins with the electron spins of paramagnetic Fe ions. Because T1 caused by this mechanism is proportional to the sixth power of the distance between the nucleus and paramagnetic ion, such mechanism is effective only in the case that paramagnetic ions are attached to the graphene sheets rather than existing as a separate phase in the compound. It allows us to conclude that the paramagnetic iron ions are anchored to the graphene sheets and presumably form iron-graphene charge-transfer complexes contributing to the 13C spin−lattice relaxation. A similar situation is also observed for nanographene samples. NGr sample shows T1 = 49 s, while Fe-doped NGrFe sample reveals T1 = 4.9 s (Table 1). Such reduction in T1 after iron doping definitely indicates that paramagnetic Fe ions attach the graphene sheets. 1 H NMR measurements of graphene samples under study show a broad signal attributed to the carboxyl (COOH) and hydroxyl (C−OH) groups partially terminating the graphene edges. 3.3. EPR. 3.3.1. RT EPR Spectra and Electron Spin Relaxation. The general view of the RT EPR spectra of LGr, LGr-Fe, and NGr samples recorded with a magnetic field scan of 0.9 T demonstrates narrow intense signals within the g = 2.00 region (so-called S = 1/2 species). No traces associated with Fe ions were observed in LGr-Fe. NGr-Fe reveals a weak broad (ΔHpp > 50 mT) line with g ≈ 2.0. Figure 6 shows EPR spectra of the intense g = 2 signals recorded with 0.06 mT scan. LGr contains paramagnetic species characterized by an asymmetric line shape, ΔHpp = 2.85 ± 0.05 mT and g = 2.0048 ± 0.0002, and the density of EPR observable spins NS = 3.7 × 1018 spin/g; see Figure 6a, black trace. LGr-Fe shows superposition of two signals: broader (ΔHpp = 4.1 ± 0.1 mT) with g = 2.0029 ± 0.0005 and narrow (ΔHpp ≈ 0.6 mT) with g = 2.0021 ± 0.0002 (Figure 1a, red trace). The latter signal at RT is well observed in the second derivative of the EPR absorption spectrum. Total density of the observable spins in LGr-Fe is NS = 1.3 × 1018 spin/g. The EPR spectrum of NGr represents the same superposition of two lines: broader with ΔHpp = 4.9 ± 0.1 mT and g = 2.0012 ± 0.0005 and well distinguishable even at RT narrow with ΔHpp ≈ 0.4 mT and g = 2.0019 ± 0.0002; see Figure 1b, black trace. NS in NGr is found to be 1.1 × 1018 spin/g. NGr-Fe sample at RT shows no observable narrow EPR signal in the g = 2.00 region. The only signal observed at high receiver gain is a very broad signal hardly distinguishable from the background; see Figure 6b, red trace. We note that no EPR signal saturation has been reached within the working microwave power range (up to 200 mW), indicating very short spin−spin and spin−lattice electron relaxation times (TeSL, TeSS < 10−9 s). 3.3.2. Temperature-Dependent EPR Spectra. Low temperature EPR spectra as well as the temperature dependencies of the EPR spectra parameters within the low temperature range are extremely useful for the correct attribution of the observed EPR signals. 3.3.2.1. LGr. The general view of low temperature EPR spectra of LGr shows no appearance of new signals. The same

Figure 6. RT EPR spectra of narrow signals in graphene samples: (a) LGr (black trace) and LGr-Fe (red trace); (b) NGr (black trace) and NGr-Fe (red trace) recorded at incident microwave power PMW = 2 mW, 100 kHz magnetic field modulation amplitude Amod = 0.3 mT, number of coherent acquisitions nacq = 25, and ν = 9.77 GHz. Signals are recorded using the same receiver gain, and peak intensities are normalized per unit weight.

intensive asymmetric line is observed within the entire temperature region. Low temperatures reveal that the asymmetric RT signal also contains a weak narrow (ΔHpp ≈ 0.6 mT) signal with g ≈ 2.002, upon turning to be observable at T approaching 100 K in second derivative EPR spectra and well observable in the first derivative (conventional) EPR spectra at T below 30 K; see Figure 7a,b. At low temperatures, the LGr EPR signal broadens up to ΔHpp ≈ 4 mT and then, on temperature increase, gradually narrows to ΔHpp ≈ 2 mT at RT, that is, even narrower than it was before cooling; see Figure 7c. The most interesting feature of the EPR-detected paramagnetic species in LGr is nearly temperature-independent (within the experimental error) integral intensity of the EPR signals (Figure 7d). The intensity of this signal shows very weak increase on cooling, which indicates significant deviation from the Curie−Weiss law. 3.3.2.2. NGr. Like in the aforementioned LGr, the general view of low temperature EPR spectra of NGr shows no appearance of new signals. On cooling, the broad component of the g = 2.0012 signal continuously broadens and becomes unobservable on approaching 100 K. Below 100 K the only narrow component with g = 2.0021 is observable. Its g-factor and line width remain unchanged at low temperatures. Integral intensity of the narrow line in NGr obeys the Curie−Weiss law at temperatures above 25 K; see Figure 8. 3.3.2.3. LGr-Fe. Temperature evolution of both g = 2.00 signals in LGr-Fe is similar to the evolution of the corresponding signals in NGr. Thus, on cooling, the broader g = 2.0012 signal broadens and turns to be unobservable on approaching 100 K, whereas the narrow signal with g = 2.0019 G


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Figure 7. Temperature dependencies of EPR spectra and EPR parameters in LGr: (a) first derivative EPR spectra at T = 25 K (black trace), 100 K (red trace), and 295 K (black trace); (b) second derivative EPR spectra at T = 25 K (black trace), 100 K (red trace), and 295 K (black trace), arrows point out the narrow signal with g ≈ 2.002; (c) line width as a function of temperature; and (d) integral intensity (SIN) as a function of temperature.

Figure 8. Inverse integral intensity (1/SIN) of the EPR signal (☆) in NGr as a function of temperature. Red dotted line represents best linear fit of the experimental data.

Figure 9. General view of temperature dependence EPR spectra of NGr-Fe (red trace) at T ≤ 70 K. All spectra were recorded at the same instrumental conditions: incident microwave power PMW = 2 mW, 100 kHz magnetic field modulation amplitude Amod = 1 mT, same receiver gain, and ν = 9.467 GHz. Background signal subtracted. Signals are shifted in vertical scale for better presentation. Vertical arrow in the top right corner indicates increasing temperature.

retains its resonance position, line width, and line shape. The intensity of the narrow signal obeys the Curie−Weiss law. The main difference of this iron-modified sample in comparison with both LGr and NGr is the appearance of a new broad signal in a low field region of general view spectra at T < 20 K. The peak intensity of this signal is very weak and hardly distinguishable from the background signal; however on the spectrum recorded at T = 5 K, a superposition of a broad low field wing and narrower signal with g = 4.16 ± 0.01, presumably attributed to the traces of iron ions (see next section), may be observed. 3.3.2.4. NGr-Fe. In contrast, the low temperature EPR of NGr-Fe demonstrates significant differences from EPR results obtained for the other graphene samples under study. The main distinguishing feature is the reliable observation of signals, which could be attributed to Fe ions. Figure 9 represents a general view of the spectra of NGr-Fe at temperatures below 70 K (background signal subtracted). The spectrum recorded at T = 5 K clearly indicates the presence of at least two signals: a very broad (ΔHpp ≈ 0.2−0.3 T) signal with g = 1.98 ± 0.01

having an intense low field wing and a narrow (ΔHpp ≈ 25 mT) asymmetric signal with g = 4.16 ± 0.01, presumably attributed to Fe2+ and Fe3+ ions, respectively. The integral intensity of the low field wing of the broad signal decreases fast on heating, and this part of the spectrum becomes practically unobservable above 35 K. On the other hand, the g = 4.16 feature may be observed at temperatures up to 70 K. Here, it is worth mentioning that just these broad signals are observed in general view spectrum of LGr-Fe at 5 K. EPR spectra of NGr-Fe recorded within the g = 2 region using the 0.06 mT scan and higher incident power PMW = 20 mW reveal that at T < 70 K a weak narrow (ΔHpp ≈ 0.5 mT) singlet becomes observable. This signal has an asymmetric line shape and g = 2.0019. Figure 10 shows low temperature dependencies of inverse integral intensities of Fe-related broad (○) and narrow (red ☆) signals H


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ions. This effect is much stronger in NGr-Fe, indicating a 6.6fold larger amount of Fe ions in NGr as compared to LGr. The next question is on iron ions location, that is, whether they are bound to graphene sheets or are present in a material as a separate phase. This problem was solved by our 13C NMR relaxation measurements showing significant acceleration of the 13 C spin−lattice relaxation under Fe doping. This finding shows that Fe ions are anchored to the graphene sheets (presumably forming Fe−graphene complexes) and establish an additional fast relaxation channel via electron−nuclear spin−spin interaction. We note that the nuclei at the immediate vicinity of the paramagnetic defect experience a noticeable magnetic field produced by unpaired electron spin. Therefore, they have strongly shifted Zeeman frequencies and severe line broadening and do not take part in nuclear magnetic resonance. Thus, one can conclude that the NMR spectra in a magnetically diluted system are little informative. In such a case, the information is mainly received from the relaxation measurements. The matter is that while nuclei located very close to the paramagnetic ion (say, inside a sphere of a radius δ) are removed from the resonance line, the other ones undergo relaxation due to the electron−nuclear dipolar coupling. Herewith the spin magnetization of the distant nuclei in a rigid diamond lattice is spatially transferred to paramagnetic impurities by means of the mutual flipping of like neighboring nuclear spins due to the dipole− dipole interaction terms of I+i I−j type (this effect is called spin diffusion).11 The larger reduction of the spin−lattice relaxation time in NGr-Fe as compared to LGr-Fe is evidently caused by a larger amount of Fe ions that anchored the edges of the nanographene sheets as compared to those that anchored the edges of micrographenes. Finally, EPR data allow us to distinguish between different paramagnetic species and to indicate that the iron ions are attached to the graphene edges. In fact, RT EPR does not detect any traces of paramagnetic transition metal ions in both pristine and Fe-doped graphene samples; in other words, no direct spectroscopic evidence of iron doping was found at RT. On the other hand, it is widely accepted that paramagnetic iron ions, especially non-Kramers Fe2+ ions, are quite problematic objects for observation by RT X-band EPR. The combination of strong anisotropy of zero-field splitting and fast electronic relaxation rates, significant line broadening, and low abundance may lead to very weak broad lines in the X-band EPR spectra of polycrystalline samples, which are practically indistinguishable from background signals. However, analyzing EPR spectra of S = 1/2 EPR species in Fe-doped spectra against their counterparts in the spectra of initial nondoped graphene samples, one can find convincing indirect evidence for the presence of magnetic iron ions in Fe-doped graphenes. Figure 6 and Table 1 demonstrate that on Fe-doping of the graphene samples, two main effects occur: (a) reduction of the density of the EPR observable S = 1/2 species and (b) significant broadening of the main (the most intense) line. The LGr-Fe sample shows an almost 3 times NS reduction and 1.25 mT line broadening, whereas in NGr-Fe initial broadening of the initial signal does not allow detection of the S = 1/2 species at all. Reduction of the integral intensity of S = 1/2 species signals may be interpreted in terms of quenching of the edge spins due to their coupling with adjacent Fe ions, which causes changes in a valence state of those ions. Alternatively, the aforementioned reduction may occur due to the dipole−dipole interaction of the S = 1/2 species with magnetic Fe2+ and/or Fe3+ ions located in close vicinity to the graphene sheet’ edges. The

Figure 10. Normalized inverse integral intensity (1/SIN) of the EPR signals in NGr-Fe as a function of temperature: ○, Fe-related broad signal; red ☆, narrow (ΔHpp ≈ 0.5 mT) signal. Blue dashed line represents best linear fit of the experimental data at higher temperatures.

observed in NGr-Fe. It is clearly seen that below 25 K, the inverse integral intensity of the broad signals deviates from the Curie−Weiss behavior. This effect is even more pronounced for the narrow signal.

4. DISCUSSION Temperature dependencies of the magnetic susceptibility of the pristine micro- and nanographenes produced by microwaveenabled method reveal two contributions. Diamagnetic contribution characteristic of graphite-like carbon systems dominates in a wide temperature range from RT to ∼30−40 K. Because diamagnetism is caused by a superposition of ring diamagnetic currents in hexagons of graphene sheets and nondissipative diamagnetic edge current flowing in the narrow strip along the sheet/domain periphery, its magnitude should depend on the size of ordered, defect-free graphene domains. This is obtained in our experiments showing |χ(LGr)| > |χ(NGr)|; that is, the larger are the perfect domains, the larger is the diamagnetic contribution to the susceptibility. As suggested by Pinnick who studied turbostratic carbons and graphitic crystals,13 an increase in susceptibility with increased lateral particle size in the range from ∼3 to ∼20 nm is due to an increase in the filing of the π band, which brings the Fermi level into a region of higher curvature of the energy surface. A similar mechanism seems to be relevant to the graphene samples under study. Moreover, the influence of the edge states provides a peak in density-of-states (DOS) in the vicinity of the Dirac contact point. Reduction the DOS peak related to edge states leads to the suppressing of the paramagnetic contributions of Curie and Pauli types from them and increasing the total diamagnetism. The closeness of the diamagnetic contributions to the magnetic susceptibility in LGr to that of bulk graphite indicates the micrometer-sized graphene samples under study are of good quality and defect-free domains of about 20 nm diameter. At temperatures below 30 K, the Curie-like paramagnetic term is dominating. This intrinsic paramagnetism is mainly caused by the unpaired π-electron spins located on the graphene edges. No ferromagnetic behavior in all samples under study was obtained down to T = 4.5 K. Paramagnetic contribution is considerably increased under iron doping of the samples, thus being evidently caused by the paramagnetic Fe I


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in the graphene samples under study to the same types of defects. Indeed, for all of the samples, g-factors of the weak narrower lines as well as the g-factor for the intense broader component in NGr coincide (within the experimental error) with the same parameters in defective nano-onions (g = 2.0019 and 2.0012, respectively).17 Moreover, temperature dependencies of the signal intensities for the corresponding species obey the same rules as reported for defective nano-onions. Thus, the “☆” in Figures 8 and 10 show that the narrower component not only obeys the Curie−Weiss law at temperatures above 25 K, but also deviates from this behavior at lower temperatures; see, for instance, Figure 9b in ref 17. However, despite the aforementioned similarities, the broader component in LGr and LGr-Fe demonstrates some differences from that observed in NGr. First, these signals have significantly larger gfactor values: 2.0048 in LGr and 2.0029 in LGr-Fe. Further, the broader component in LGr shows (in contrast with NGr and even LGr-Fe) less intensive line broadening on cooling: it is well observable with the whole temperature range of 5−295 K. The latter fact allows reliable measurement of the temperature dependence of this EPR line, which revealed non-Curie behavior of its EPR magnetic susceptibility at T > 25 K. We have recently observed the same effect for the edge-state spins and localized defects in large polyhedral multishell carbon particles, astralens.18 Following ref 18, the unusual properties of the EPR-active spins observed in LGr may be understood within the framework of the well-developed theory of electron resonance of localized electron spins in metals.19 According to this theory, the main factor determining all EPR features is the ratio of magnetic susceptibilities of conduction electrons χs to the localized paramagnetic centers χd: χr = χd/χs. In the cases of χr ≪ 1, the conduction electron properties will predominate, and EPR signals of localized paramagnetic centers will behave in the same manner as signals of conduction electrons spin resonance; that is, their intensities will demonstrate the Paulitype paramagnetism. When χr > 1, the properties of the EPR signal will be determined by the properties of localized paramagnetic centers, that is, the Curie-type paramagnetism. The similar, almost temperature-independent EPR signals have been observed in metallic (or semiconducting) multiwall carbon nanotubes (MWCNT) where χEPR(T) has been interpreted as the dominant, slightly temperature-dependent Pauli contribution and a small Curie component.20 Hypothesis on fulfilling the χr ≫ 1 condition in LGr assumes the presence of a strong exchange interaction between itinerant carriers and edge localized spins responsible for the g = 2.00 EPR signals. Such an interaction also leads to the shift of the resonance field for EPR observable spins due to the effective hyperfine field induced by conduction electrons.21 This effect explains positive g-shifts of the broader components in both LGr and LGr-Fe. Another indication of the presence of strong exchange with conduction electrons is weak sensitivity of the broader component of the EPR signal to the broadening by molecular oxygen on cooling, found in LGr. It seems that in this system, the exchange narrowing due to conduction electrons is much more effective than the dipole−dipole broadening due to the adsorbed molecular oxygen. Here, it is worth mentioning that in the iron doped LGr-Fe sample, both positive g-shift and line broadening/narrowing effects are less pronounced than in pristine LGr. These changes may indicate that Fe doping somehow spoils high conductivity of the LGr sample, causing a significant decrease in the number of conduction electrons and thus breaking the χr ≪ 1 conditions.

dipole−dipole interaction causes line broadening, which depends on distances between edge spins and Fe ions and follows the r−6 rule, and thus it is effective for quite short distances, to the order of few nanometers. Using the technique proposed in ref 14, one can estimate that the observed broadening of the EPR signal in LGr-Fe due to the doping by Fe2+ or Fe3+ ions with spin density of ∼1019 spin/g (see SQUID data in Table 1) is effective for edge spin−Fe distances of about 1.3−1.4 nm. EPR signals of edge spins located at distances shorter than 1.3−1.4 nm from Fe ions are broadened much stronger and are not observable at all. The latter, most probably, is the main factor of the 70% reduction of the initial edge spin EPR signal observed in the nondoped LGr sample. When applying the same concept to the nanographene samples, one should keep in mind both the higher density of doping Fe ions (∼8 × 1019 spin/g) and the shorter length of individual particle edges. In combination, it causes extremely effective broadening of the EPR signals of all edge spins and observation of a very broad (∼50 mT) and weak background-like signal in this sample. In addition to the aforementioned concern, low temperature EPR spectra provide direct evidence of the presence of paramagnetic Fe ions in the Fe-doped samples. Indeed, the EPR spectrum in Figure 9 recorded at T = 5 K may be interpreted as superposition of the relatively narrow g = 4.16 signal, which may be assigned to the middle Kramer’s doublet transitions of a rhombically distorted high-spin Fe3+ complex15,16 and very broad signal with g = 1.98, having a well pronounced low field shoulder. The latter signal could be attributed to both Fe2+ and another Fe3+-containing complex. Unfortunately, weak peak intensity does not allow reliable attribution of this signal. However, analyzing the temperature dependence of the inverse intensity of the Fe-related signals in Figure 10, one can see a deviation from the Curie−Weiss law at temperatures below 25 K. This deviation indicates that on cooling, the intensity of the broader asymmetric component grows faster than the g = 4.16 component, attributed to Fe3+ ions, which allows us to assume that the broader signal is due to Fe2+ ions, the EPR of which is observable only at lowest temperatures. Thus, in the NGr-Fe sample, EPR evidences the presence of iron ions in both 2+ and 3+ valence states. Because the same (but very weak) Fe-related signals were observed in LGr-Fe, the above hypothesis on Fe ions is applicable to the LGr-Fe sample as well. Analysis of the RT and temperature-dependent EPR data for S = 1/2 species allows one to draw some conclusions on the origin of these species. In general, EPR signal features for all of the graphene samples under study are quite similar: they consist of intense broader (several mT, except NGr-Fe) and weak narrower (ΔHpp ≈ 0.4−0.6 mT) components. In LGr-Fe and NGr, the narrower signal with g = 2.002 is observed at RT, whereas in LGr and NGr-Fe the same signals appear to be detectable on cooling. Cooling leads to very fast broadening of the intense component, which in LGr-Fe and NGr practically disappears on approaching 100 K. Below 100 K, in all of the samples excluding LGr, the only narrow component is observed. Such a set of EPR signals and their behavior have been recently reported for air-rich defective nano-onions,15 where the intense broader component has been attributed to the π-electronic edge-localized spins interacting with molecular oxygen, and the narrower one to some stable defects in graphene structures, which for some reasons are nonsensitive to the oxygen environment. It allows us to assign S = 1/2 species J


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Figure 11. Most probable schemes of fixation of Fe2+ and Fe3+ ions to the graphene edges.

magnetic susceptibility indicates good quality of the micrometer-sized graphene samples under study. We show that the paramagnetic properties of the studied graphene samples are caused by intrinsic edge-state and Fe-introduced paramagnetism. The attachment of Fe ions to the edges of graphene sheets is confirmed by an increase in the 13C spin−lattice relaxation rate and reduction of the EPR signal coming from unpaired electron spins of these states after iron doping. These effects are caused by the interaction of paramagnetic Fe ion spins with carbon nuclear spins and electron spins of the edge states, respectively. The concentration of Fe ions attached to the edges of NGr-Fe is ∼6.6 times larger than that in LGr-Fe. This finding reveals a noticeable capability of nanographene for the fixation of Fe ions on the periphery of graphene sheets terminated by various oxygen-containing groups and atomic hydrogen.

Thus, the attachment of Fe ions to the edges of graphene sheets is confirmed by an increase in the 13C spin−lattice relaxation rate and reduction of the EPR signal coming from unpaired electron spins of the edge states after iron doping. XPS measurements of the samples under study reveal carboxyl, carbonyl, and hydroxyl groups. Herewith the amount of oxygen-containing functional groups on the NGr periphery is larger than that in LGr.3 We suggest the active (>CO) and (>C−OH) sites, located on the edges of graphene nano- and microsheets (not on the basal planes), are responsible for the fixation of the Fe2+ and Fe3+ ions. Figure 11 shows possible fixation of bivalent and trivalent iron ions to the zigzag graphene edges by means of different functional groups, mostly >CO, −OH, or their combinations, interspersed with >CH terminated sites. Additional coordination of iron ions may be done by hydroxyl groups, the presence of which is most likely during prolonged storage of the material in air. A feature of this model is the presence of sites that fix Fe2+ ions through the pairs of the neighboring carbonyl groups (Figure11a), although the sites with localized Fe3+ ions are also possible by additional −OH group coordination (Figure 11b). Any of these ions can be also fixed on the edge of one of the >CO groups or in coordination with one or two hydroxyl groups (Figure 11c). In our opinion, the aforementioned schemes of ion fixation are the most likely to occur from at least 10 possible schemes, including various combinations with participation of carboxyl groups and those of the neighboring graphene sheets. Herewith the content of iron ions fixed by NGr is 6.6 times larger than that of LGr, although the length of the external periphery edges of graphene flakes for NGr and LGr differs by 50−100 times according the lateral size measurements. It means that the periphery of graphene sheets is not completely iron-functionalized. It may also mean that an increase in the number of the zigzag edge sites on transition from LGr to NGr is not too large. Anyhow, the observed modification of the edges in NGr-Fe reveals a noticeable capability of nanographene for fixation of Fe ions on the periphery of graphene sheets terminated by various oxygen-containing groups and atomic hydrogen.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +972 8 6472458. Fax: +972 8 6472904. E-mail: pan@ bgu.ac.il. *Phone: +7 812 9045298. Fax: +7 812 2971017. E-mail: [email protected]ffe.ru. Notes

The authors declare no competing financial interest.

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5. CONCLUSION Summarizing, we undertook a combined magnetic susceptibility, NMR, and EPR investigation of the structure and magnetic properties of pristine and Fe-doped micro- and nanographenes produced by direct microwave-enabled technique, which allows production of nearly defect-free micro- and nanographene sheets. Significant orbital contribution to the K


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Structure and Magnetic Properties of Pristine and Fe-Doped Micro

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