Correlated Optical and Magnetic Properties in Photoreduced

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Correlated Optical and Magnetic Properties in Photoreduced Graphene Oxide Takaaki Taniguchi, Hiroyuki Yokoi, Masaki Nagamine, Hikaru Tateishi, Asami Funatsu, Kazuto Hatakeyama, Chikako Ogata, Masao Ichida, Hiroaki Ando, Michio Koinuma, and Yasumichi Matsumoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509399x • Publication Date (Web): 05 Nov 2014 Downloaded from http://pubs.acs.org on November 19, 2014

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The Journal of Physical Chemistry

Correlated Optical and Magnetic Properties in Photoreduced Graphene Oxide

Takaaki Taniguchi*a,b, Hiroyuki Yokoi*a,b, Masaki Nagaminea, Hikaru Tateishia,b, Asami Funatsu a,b, Kazuto Hatakeyama a,b, Chikako Ogata a,b, Masao Ichidac, Hiroaki Andoc, Michio Koinuma a,b, Yasumichi Matsumotoa,b a

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami ,

Kumamoto 860-8555, Japan; E-mail: [email protected] b c

JST, CREST, 7 gobancho, Chiyoda-ku, Tokyo 102-0075, Japan

Department of Physics, Konan University, Okamoto, Higashinada-ku, Kobe 658-8501,

Japan

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ACS Paragon Plus Environment


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Abstract: Optical and magnetic properties of graphene oxide (GO) have been intensively investigated because of the promising applications of GO-related materials in various technical fields. So far, the optical and magnetic properties of GO have been discussed independently. However, localized electronic states in reduced GO may simultaneously add optical transitions and spin moments in sp2 nanodomains in GO nanosheets. In the present study, the structural, optical, and magnetic properties of graphene oxide (GO) photoreduced in an aqueous solution are correlated on the basis of experimental and theoretical investigations. Experimental observations show that photoreduction

leads

to

enhancement

of

visible

absorption,

quenching

of

photoluminescence, and emergence of magnetism. Detailed spectroscopic and microscopic characterizations indicate the presence of photoreduction-produced basal plane C–H bonding and carbon vacancies. Ab initio calculations suggest that the presence of these defects in sp2 nanodomains results in singly occupied molecular orbital levels in the π–π* gap to afford enhanced visible to near infrared (NIR) absorption and emergence of magnetism, which is consistent with the experimentally observed change in the optical and magnetic properties of GO by photoreduction. Enhancement of NIR emissions observed in shortly photoreduced GO and their extinction found in longer photoreduced GO are explained with integrating the theoretical calculations and time-resolved fluorescence measurements. The correlation among structural, optical, and magnetic properties, highlighted for the first time, could help accelerate the development of open-shell nanographene devices with concurrently tunable electrical, optical, magnetic, and electrochemical properties.

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1. Introduction Chemically derived graphene oxide (GO) has emerged as a multifunctional material exhibiting great potential for practical applications in numerous technical fields. Single-layered GO sheets can be obtained in a high yield in a facile and cost effective method by the exfoliation of graphite oxide1,2. The subsequent reduction of GO enhances the electrical conductivity owing to deoxygenation3. Reduced GO (rGO) has been widely used in transparent conductive films4, electrodes5, and electrocatalysts6. Optical and magnetic studies have been conducted to reveal the complex nature of the electronic states of GO-related materials. For example, GO exhibits absorption in the ultraviolet and visible regions due to a π–π* transition in sp2-dominant nanosized islands (sp2 nanodomains) isolated by oxidized sp3-carbon backbones7,8,9. Compared with GO, rGO has increased visible absorption, which has been attributed to restoration of the π network7,9 or increased charge carrier concentration10,11. GO displays photoluminescence (PL) bands in the range of 500–800 nm, where PL in this range can originate from ensemble emission from sp2 nanodomains of various sizes7,12. In contrast, thermally or photothermally reduced rGO shows fluorescence between 350 and 500 nm13,14. Eda et al. have reported that sp2 fragments (sp2 nanodomains smaller than those in as-prepared GO sheets), which are produced during the reduction, are responsible for the blue-green luminescence13. The optical tunability of GO offers advantages for applications such as optical DNA sensing and fluorescent bioimaging8. Reduction also strongly influences the magnetic properties of GO. So far, magnetic properties of rGO obtained by various reduction methods, including hydrazine reduction15, thermal reduction16, combined hydrazine17, and Birch reduction18, have been experimentally investigated. In these studies, conversion of diamagnetic GO to

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rGO that is paramagnetic or even ferromagnetic at RT was demonstrated. The formation of edges, vacancies, doping of hetero-atoms, and chemisorption were considered to be spin-moment sources activating magnetism in GO on the basis of theoretical works on graphene; numerous reports already predicted ferromagnetic ordering could exist among various defects on infinite graphene sheets, such as vacancy19, topological defects20, and hydrogen chemisorption21 or edges22. Optical and magnetic properties of rGO-related materials have been discussed independently in the cited studies. However, reduction-introduced electronic states may simultaneously influence optical and magnetic properties of rGO. Unlike infinite graphene sheets with a zero band gap, sp2 nanodomains in GO sheets have a HOMO-LUMO gap. Thus, stable radical formation via reduction of GO, pointed out recently23-25, potentially adds both optical transitions and spin moments as a result of introduction of singly occupied molecular orbital (SOMO) levels between HOMO and LUMO levels in sp2 nanodomains. Open-shell nanographene structures have great potential to develop molecular spin devices such as organic magnetic semiconductors, spin batteries26,27 and radical light-emitting diode28. Thus, searching correlation between optical and magnetic properties would not only provide new insights into the physical properties of GO-derivatives, but also could open new avenues for designing graphene-based spintronics. In the present study, we investigated ultraviolet-visible absorption, steady-state and time-resolved PL in the visible to near infrared (NIR) range, and magnetic properties of photoreduced GO. With photoreduction methods, we could progressively reduce GO by increasing irradiation time without using additional chemicals29,30. Thus, this method was suitable to monitor reduction-dependent properties of GO. In addition to the optical

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and magnetic measurements, detailed spectroscopic and microscopic characterizations were performed to seek origins of the photoreduction-induced changes in the optical and magnetic properties. Furthermore, Ab initio calculations were performed in order to reveal whether there is a possible correlation among the structural, optical and magnetic properties in defective sp2 nanodomains. 2. Results and discussion 2.1. Effects of photoreduction on optical and magnetic properties First, we investigated the effect of GO chemical states on optical properties. GO as-synthesized using the Hummers method31 was photoreduced to produce rGO samples. The black-colored rGO dispersion obtained after photoreduction for 6 h maintained high colloidal stability (see Supporting Information, Figure S1), while photoreduction for 40 h resulted in the precipitation of hydrophobic rGO sheets. Figure 1a displays the ultraviolet-visible absorption spectra of as-prepared GO and rGO aqueous dispersions. The absorption intensity in the visible range increased with an increase of photoirradiation duration, where the optical energy gap estimated by the Tauc plots (Figure 1b) shifted toward low energy side from 2.9 eV to 1.5 eV. Raman spectroscopic analysis (Figure 1c) showed that the integrated intensity ratio of the D band at ~1350 cm−1 to the G band at ~1600 cm−1 (ID/IG) slightly increased after photoreduction for 6 h. This means that the average size of sp2 nanodomain in GO was almost unchanged by photoreduction in an aqueous solution. Thus, the remarkable increase in visible absorption was not attributed to the growth or nucleation of sp2 nanodomains, but it is likely to be attributed to the formation of deep levels inside the π–π* gap, which could cause the reduction of the optical energy gap.

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PL measurements were performed to further investigate the effect of photoreduction on GO electronic states. Figure 1d shows typical PL spectra in the visible region for GO and rGO dispersions. Absorbance of the GO and rGO samples was adjusted to be almost the same above 500 nm by dilution of as-obtained GO and rGO dispersions with water. Thus, the PL spectral shape and intensity of these samples are compared without a contribution from emission reabsorption. Unreduced GO produced a broad PL band at ~650 nm due to the π−π* transition, corresponding well with literature report7,12. As photoreduction progressed, PL was progressively redshifted and weakened. Note that PL-quenching and subsequent enhancement, observed in Sokolov’s work using a continuous 405nm-laser for photoreduction32, was not observed in our study, demonstrating that resultant rGO electronic states are strongly sensitive to the relevant photoreduction conditions. It is also notable that the blue-green PL was not observed for any excitation wavelength for the rGO samples as well as samples prepared for shorter reduction duration. Thus, the sp2 fragments that would give such a PL band were not nucleated during photoreduction. Again, Raman spectroscopy indicated that the average size of sp2 nanodomains did not increase with photoreduction employed in the present study, so weakened quantum confinement that accompanies sp2 nanodomain growth was not responsible for the PL redshift. On these bases, we propose that the introduction of defect-induced deep levels redshifted and quenched PL with an increase in photoirradiation duration. The optical investigations suggested that photoreduction of GO introduced localized levels rather than nucleation and/or growth of sp2 nanodomains. If these levels come from local structures with unpaired electrons, they would afford localized spin moments and thus magnetism to GO. We investigated the influence of photoreduction on

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magnetic properties of GO using a superconducting quantum interference device. Figure 2a shows the RT magnetization-magnetic field (M–H) curves for as-synthesized GO and rGO photoirradiated for 6 h and 40 h. The as-prepared GO displayed diamagnetic behavior at RT. In contrast, a paramagnetic signal was predominantly observed for the rGO samples. As the photoirradiation time was increased from 6 h to 40 h, the positive signal increased, which indicates that more spin moments were introduced in GO with longer photoreaction times. Metal contamination during the photoreduction step can be excluded as a cause because of the absence of additional chemicals for photoreduction; in fact, the levels of magnetic transition metals, such as Fe, Co, Cu, and Ni, were below the detection sensitivity limit for inductively coupled plasma analysis for both the GO and rGO samples. Ferromagnetic response was observed after subtracting the diamagnetic and paramagnetic signals of GO and rGO samples, respectively (Figure 2b). As well as paramagnetic signal, ferromagnetic signal was generally intensified by photoreduction. Dominant diamagnetism with weak ferromagnetism from as-obtained GO flakes was also reported in the literature33. Thus, enhanced ferromagnetism by photoreduction is probably intrinsic magnetic properties of rGO. The enhanced paramagnetic behavior and the decreased saturation magnetization with an increase in photoirradiation duration to 40 h suggest that the origins of those properties should be distinguished. We tentatively assume the following scenario: i)

The density of the defects on the basal plane of rGO could be uneven.

ii) The defects in the dense areas are expected to contribute to the ferromagnetic coupling while the number of defects located on the neighboring sites is supposed to increase with increasing the density of defects. The magnetic moments among the neighboring defects are known to be canceled34, which leads to the reduction of the

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saturation magnetization in the longer photoreduced rGO. iii) The paramagnetic behavior originates from the defects in the sparse area, where the magnetic moments of the defects are isolated. With proceeding with photoreduction, the sparsely defected areas are produced more and more, which should enhance the paramagnetic behavior. 2.2. Structural analysis of photoreduced GO Local structure of rGO was investigated using C1s X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM),

13

C solid state nuclear magnetic

resonance (SSNMR), and Fourier transform infrared (FT-IR) spectroscopy (FT-IR) in order to seek origins of the photoreduction-induced changes in the optical and magnetic properties. Figure 3 displays C1s XPS spectra for as-prepared GO and rGO obtained by photoreduction. Note that the broad peak at ca. 285 eV was singly assigned to the C=C bond in most of the previous works on GO. However, deconvolution of this peak is essential for understanding the C1s XPS spectra of GO35. This is because the full width at half maximum (FWHM) is too large to assign the band to a single component (1.3 eV) for GO, considering much smaller FWHM of the C=C peak from highly ordered pyrolytic graphite (0.8 eV) measured with the same experimental conditions36. Therefore, recently, we have developed deconvolution of C1s XPS spectra in order to identify chemical states of GO, using reference samples with specific oxygen function group (OFG), C-H, C-C, or C=C bonding states36,37. In particular, our approach allows for the deconvolution of the peak at ca. 285 eV into three peaks from the non-oxygenated groups, including the sp3 C–C bond at 285.5 eV, the basal plane C–H bond at 285 eV, and the sp2 C=C bond at 284.5 eV. Results of a theoretical study by Yamada et al. generally support the results of our peak deconvolution method38, while

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their work suggests that the peak at 285 eV might also be due to the presence of carbon vacancies. According to the C1s XPS analysis of GO and a series of photoreduced rGO samples (Figure 3), deoxygenation is mainly due to the decreased number of epoxy groups. This is because the epoxy groups preferentially located on the basal plane have the lowest stability among the OFGs. Along with the decomposition of epoxides, the relative peak intensity at 285 eV increased remarkably with an increase in photoirradiation duration up to 40 h. Thus, XPS suggests that the photoirradiation decomposed epoxy groups to form basal plane C–H bonds and/or carbon vacancies. Recently, using electron paramagnetic resonance (EPR) measurements, Hou et al. demonstrated the presence of stable radicals in photoreduced GO23. Thus, the presence of carbon vacancy sites with dangling bonds, i.e., radicals, is a realistic supposition for our samples. Presumably, the sufficiently large π-conjugation in rGO could offer the stability for unpaired electrons24,39 Vacancy formation in rGO was confirmed with TEM. TEM observation of rGO photoreduced for 40 h generally displayed amorphous like images due to the highly disordered structures and agglomeration (Figure 4a). Nevertheless, careful high resolution TEM (HRTEM) observation revealed nanoscale region with graphene honeycomb lattice (Figure 4b). The observed structure was highly defective, where vacancy defects as well as lattice deformations were included. Photoreduction for 40 h increased approximately 1.5 fold the ID/IG ratio (Supporting Information, Figure S2) presumably because of the formation of these defects. Figure 5a presents the 13C SSNMR spectra of as-obtained GO and rGO photoreduced for 40 h. The peaks shown in the spectrum are generally assigned to the functional groups previously reported for GO40. It is evident that the relative peak intensity of

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epoxides at 70 ppm decreased significantly by photoreduction, while a relatively sharp peak corresponding to C–H bonding was detected at 40 ppm for the rGO sample41,42. These results are well accord with the XPS analysis. It is also notable that C=C peak intensity in the 100–140 ppm range broadened with photoreduction. According to the literature43, the presence of paramagnetic carbon centers associated with dangling bonds causes broadening of the resonance lines. sp2 signal was not intensified after photoreduction, which indicates that sp2 domains were not restored. Note that the COOH peak was most strongly detected after photoreduction likely due to the highest stability. Thus,

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C SSNMR further supported the supposition that photoreduction

induces vacancy sites and the detected spin moments arises from carbon-centered radicals in rGO, not metallic impurities. Figure 5b displays the IR absorbance spectra of as-obtained GO and GO photoreduced for 40 h. Several distinct vibrational modes of various types of bonding states in GO can be readily identified44: hydroxyls (C–OH, 950–1200 cm−1), epoxides (C–O–C, 1200– 1300 cm−1), and carboxyl, carbonyl, and ketone groups, as well as sp2 bonding (COOH, C=O, C=C, water bending modes, 1300–1800 cm−1), while higher wavenumber peaks are assigned to sp3 C–H (ca. 2900 cm−1), sp2 C–H (ca. 3130 cm−1), and OH (ca. 3400 cm−1)42. In general, the peak intensities significantly decreased overall because of deoxygenation and dehydration with reduction. Indeed, peaks assigned to sp3 C–H and sp2 C–H stretches were detected at 2910 cm−1 and 3130 cm−1, respectively43,45, for the rGO sample, which suggests that the sp3 C–H and sp2 C–H are situated on the basal plane and vacancy (or edge) sites, respectively. Thus, FT-IR results provide further evidence for the presence of basal plane C–H bonds in the rGO sample, and FT-IR additionally suggests that vacancy sites could be passivated partially by hydrogen

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atoms. 2.3. Ab initio calculations Ab initio calculations were carried out to investigate how the formation of C–H bonding and carbon vacancies affects the optical and magnetic properties of a sp2 nanodomain. Hydrogenation effects were investigated for two cases of an adatom on the basal plane and terminations of a vacancy in the basal plane. We chose (C4H)24, a hexagonal sheet with hydrogenated zigzag-edges (GH0, Figure 6a), as a model material. The effect of a hydrogen adatom on the basal plane was examined for the structure where the adatom is located on the carbon atom labeled 1 (GH1) in Figure 6a. In another case, the carbon atom was removed to form a vacancy (GVH0) in the structure and one to three hydrogen atoms were attached to carbon atoms at the pore edge one by one from the position a to b (GVH1-3) as indicated in Figure 6b. We have analyzed absorption spectra and difference of spin densities in order to examine the optical and magnetic properties of these nanodomains46. The optimized structures of GH0-1 and GVH0-3 are presented in Figure 7. The spin-polarized state is found to be more stable than the spin-unpolarized state for each of the defected structures as shown in Table 1. The energy levels calculated for these structures are shown in Figure 6c (GH0), 6f (GH1), 6e (GVH0), and Figure S3a-c (GVH1-3), Supporting Information), respectively. The highest occupied π states (HOPSs) and lowest unoccupied π states (LUPSs) are also indicated in the figures. One would notice that all the defected domains have split HOPSs and LUPSs due to their lower symmetry. Indeed, these defects introduce mid-gap states (SOMO states) between HOPSs and LUPSs, where the mid-gap states were found to split into a filled spin-up state and an unoccupied spin-down mid-gap state in the hydrogenated structures. The

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calculation of the partial charge density of each state has revealed that the mid-gap states are localized to the defects (Figure S4, Supporting Information). The degree of localization of the mid-gap states for GVH0 and GVH1 is found to be smaller, which is related to the calculated results that the mid-gap states have energies closer to those of HOPSs in GVH0 and GVH1 than in the other defected structures. Figure 8 displays absorption spectra calculated for these structures. Average of extinction coefficients κxx and κyy, where the x and y axes were taken in parallel to the basal plane, was employed, reflecting the typical features of their absorption properties. The main absorption peak for each structure is recognized around 1.4 eV, corresponding to the HOPS-LUPS transition. The peaks for all the defected structures are broadened due to the split of HOPSs and LUPSs. A more remarkable feature is the emergence of a side peak or a tail on the lower energy side of the main peak for each of the defected structures as indicated with arrows in Figure 8, demonstrating that partial overlap of the wavefunctions between the HOPS and the mid-gap states or between the LUPS and the mid-gap states, i.e. the origin of the side peaks or the tail is attributed to the mid-gap states. These calculated results would connect our two experimental observations for photo-reduced GOs consistently: increase of optical absorption in the visible light region and increase of carbon vacancies and C-H bonds. Furthermore, all the hydrogenated structures are found to be magnetized as indicated in Table 1. Isosurface plots of the spin density for the hydrogenated structures demonstrate clearly in Figure 7 that the spin-up charges distribute mainly around the hydrogenated sites and exceed the spin-down charges on the whole nanodomains. We have also observed magnetization of photo-reduced GOs as mentioned above. Integrating all the calculated and experimental results, it is possible to explain the photo-reduction induced modification of both the

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optical and magnetic properties in terms of hydrogenation of sp2 nanodomain surface and vacancies without contradiction. 2.4. NIR-PL properties of GO and photoreduced GO Finally, we investigated the NIR PL properties of GO and photoreduced GO by steady-state and time-dependent measurements. Recently, Eng et al. observed a broad NIR luminescence band at ca. 930 nm in highly hydrogenated rGO with RT ferromagnetic properties18. They tentatively suggest that strong luminescence in NIR regions is due to formation of high concentrations of deep levels inside the gap. Our calculations propose that the NIR emission is produced by optically allowed transitions through the mid-gap levels. Figure 9a displays a NIR PL matrix for fresh GO, and GO photoreduced for periods ranging from 1 h to 6 h. Unreduced GO exhibits mainly the tail of the visible band PL in this region. PL bands at ~930 nm and 1050 nm gradually appear with increasing photoreduction time, while total PL intensity is significantly decreased (Figure 9b). The peak position of the NIR PL is similar to that reported in Eng’s work18, indicating that the NIR PL from the photoreduced and Birch-reduced rGO samples have the same origin. The excitation band of the NIR PL closely overlaps with the visible emission in the spectra of the unreduced GO sample (Figure 9a). This indicates that the initial excitation state is identical for visible and NIR PL. On this basis, we can infer that the photogenerated electrons and holes in sp2 nanodomains are first produced in HOPS and LUPS levels, respectively, and then exciton recombination occurs through the mid-gap levels to yield the NIR emission. Although Birch reduction enhanced the NIR PL intensity in Eng’s study18, enhancement of NIR emissions was only observed in shortly photoreduced GO (1 h) and their quenching was found in longer photoreduced GO. The time-resolved

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fluorescence decay curves at 950 nm show that the photoreduction of GO decreases the decay time constants (Figure 9c and Figure S5, Supporting Information), suggesting that photoreduction produces deficiencies that give nonradiative paths. We speculate that these deficiencies could hamper ferromagnetic ordering, e.g. they weakened carrier-mediated interaction between magnetic moments in sp2 region. As a result, unlike rGO obtained by Birch reduction18, the photoreduced rGO samples were supposed to show tiny ferromagnetic signal. Although these deficiencies have not been identified yet, we assume that they located at boundaries or edges of sp2 nanodomains. In that case, their formation could inhibit the nucleation and growth of sp2 nanodomains. Considering the real GO structures, many factors, such as sizes, configurations, concentrations, and interactions of sp2 nanodomains and sp3 regions, should have complex influences on the magnetic properties as well as electrical and optical properties. In an our ongoing study, we have been developing theoretical models considering these factors to make a clearer link between experimentally and theoretically derived reduction-dependent physical and chemical properties. 3. Conclusion Our combined experimental and theoretical study has demonstrated that the optical and magnetic properties of sp2 nanodomains in GO are concurrently tunable. Experimental observations show that photoreduction leads to enhancement of visible absorption, quenching of PL, and emergence of paramagnetism/ferromagnetism. Detailed characterizations prove that basal plane C–H and carbon vacancy sites are typical local structures induced by photoreduction. Ab initio calculations demonstrate that such open-shell graphene structures introduce SOMO levels in the π–π* gap in sp2 nanodomains, which would lead to optical absorption, NIR PL, and magnetism.

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Steady-state NIR PL spectroscopy propose the photo-generated electrons and holes in sp2 nanodomains are first produced in HOPS and LUPS levels, respectively, and then exciton recombination occurs through the SOMO levels to yield the NIR emission. Strong quenching of PL and decrease of the decay time constants by photoreduction suggest formation of deficiencies, which could hamper ferromagnetic ordering. Simultaneously tunable optical and magnetic properties open new avenues for the future development of GO-multifunctional materials. 4. Experimental Section Synthesis of GO: graphite powder (98%) was purchased from Wako. The graphite powder was oxidized by Hummers method using NaNO3, H2SO4, KMnO4, and H2O2 solutions. The resulting mixture was washed several times with distilled water. The resulting graphite oxide was suspended in distilled water by bath sonication for 2 h, and the suspension was centrifuged at 20,000 rpm for 30 min to remove unexfoliated GO. Photoreduction of GO: in a quartz cell (400 mL), 50 mL of the as-obtained GO aqueous suspension was placed. The GO colloid was irradiated with light from a 500 W Hg lamp (Ushio, SX-UI500XQ) for 15 h. After reduction for 1, 3, and 6 h, 10 mL samples of the suspension were collected. Characterization: XPS measurements were performed under vacuum (>7–10 Pa) using a spectrometer (Thermo Scientific Sigma Probe) equipped with a monochromatized X-ray source (1486.6 eV). Electrons emitted from the samples were detected by a hemispherical energy analyzer equipped with six channeltrons. The overall energy resolution for XPS was below 0.55 eV (on Ag 3d3/2 with a pass energy of 15 eV). The GO solutions were also analyzed by UV–Vis absorption spectroscopy (JASCO, V-550). For absorption spectra measurements, as-obtained GO and as-produced rGO dispersions

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were 20 time-diluted by adding water. RT-PL spectra in the 500–730 nm PL region and the 900–1300 nm region were obtained using a spectrofluorometer (Jasco FP-6500) equipped with a 150 W Xe lamp and a Horiba JobinYvon Fluorolog-3 spectrofluorometer equipped with a 150 W Xe lamp, respectively. Raman spectroscopy (Jasco, NRS-3100) was performed using a 532 nm excitation source at room temperature. A Quantum Design MPMSXL-5 superconducting quantum interference device

(SQUID)

magnetometer

was

used

for

magnetization

measurements.

Powder-sample was placed inside the SQUID chamber, and the field dependence of the magnetization was measured at 300K. In the TRF measurements, TCSPC was employed. GO solutions were excited at 800 nm with a Ti:sapphire laser (Spectra-Physics, Tsunami) and emissions were detected at 950 nm with a photomultiplier tube (Hamamatsu Photonics, R5509-42). The typical width of the laser pulse was 0.5 ns. The deconvolution of the time profiles of emission was performed, considering the laser pulse profiles and two exponential components with different decay times. The room temperature infrared (IR) Fourier- transform spectra were recorded on a Jeol JIR-7000 spectrometer. The products (20 mg) were thoroughly ground with 400 mg of potassium bromide powder (KBr for IR, Wako) and subjected to IR analysis. 13CSSNMR spectra were measured by using Varian UnityInova AS400. NMR frequency referencing was performed by adjusting carbon peak of adamantine to 38.5 ppm. A GO sample of 50 mg mixed with ZrO2 powder was spun at 4000 Hz using 7 mm double bearing ZrO2 rotors. High power proton decoupling of 60 db with fine attenuation of dipole r = 4000 was used only during detection period. Adamantine was used for adjusting the magic angle before experiment. (ICP) atomic emission spectroscopy was performed using a CID plasma photoemission spectrophotometer (IRIS Advantage, Nippon Jarrell Ash).

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Transmission electron microscopy was performed on an FEI Tecnai G2 F20 transmission electron microscope. Theoretical calculations: in the calculations, optimization of the structure and evaluation of the electronic states were carried out using the projector augmented wave (PAW) method47 in the framework of density functional theory with a plane wave set as implemented in the Vienna ab initio simulation package (VASP)48-51. The generalized gradient approximation (GGA) in Perdew-Burke-Ernzerhof scheme52 was used for the exchange-correlation energy functional. The projection operators were evaluated in real space. The self-consistent iterations and the optimization of atomic positions through the conjugate gradient algorithm proceeded until the changes of the total energy and the eigenvalues were both less than 1x10-7 eV per cell and the force on each atom is less than 0.02 eV/A, respectively. We set a rectangular parallelepiped unit cell with vaccum space of at least 12 angstrom around a graphene sheet to avoid interactions between sheets. The k mesh in the Brillouin zone was set to 1x1x1 to ignore the periodicity. We employed the plane wave cutoff energies of 520 and 400 eV and Gaussian smearing with the smearing width of 0.05 and 0.02 eV for structure optimization and electronic states calculation, respectively. We have analyzed absorption spectra and difference of spin densities46. As DFT calculations are known to underestimate transition energies, we discuss on the energies qualitatively.

Supporting Information. Photographs of GO and rGO dispersion, Raman spectra of rGO photoreduced for 6h and 40h, Energy levels of GVH1-3. Isosurface plots of the partial charge density of the occupied mid-gap states, NIR PL decay time of GO and

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rGO, obtained by TCSPC measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Takaaki Taniguchi. [email protected] Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami , Kumamoto 860-8555, Japan *Hiroyuki Yokoi. [email protected] Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami , Kumamoto 860-8555, Japan

Funding Sources This work was supported by Grant-in-Aid for Challenging Exploratory Research (No. 2365116).

ACKNOWLEDGMENT We express our gratitude to Prof. S. Hayami and Mr. T. Oishi (Kumamoto University) for the SQUID and NMR measurements, respectively.

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a)

0.8 0.6

0.50

b) 0.50

0.25

(αhν)1/2 (cm-1/2)

Figures and Captions

Absorbance (α）

0.4

0

0.2 GO

0.0 200

1h

600 800 3h

6h

0.25

Eg (eV) 2.9 2.3 1.9 1.5

0.00

400

600

800

1

Wavelength (nm) ID/ IG GO 1.3 1h 1.4 3h 1.3 6h 1.5

2 3 4 Photon energy (eV)

5

d) GO

PL Intensity (a.u.)

b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1h

3h 6h

1000 1250 1500 1750 2000 Raman shift

550

(cm-1)

600

650

700

750

Wavelength (nm)

Figure 1. (a) UV-Vis absorption spectra and (b) Tauc plot converted from the UV-Vis absorption spectra. (c) Raman and (d) Visible PL spectra of as-prepared GO and rGO prepared

by photoreduction for 1h, 3h and 6 h excited at 450nm. Inserted spectra in Figure 1a shows UV-Vis spectra of diluted samples for the PL measurements.

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Figure 2. (a) RT M-H curves of GO and rGO prepared by photoreduction for 6h and 40h at room temperature. (b) ΔM as a function of H in GO and rGO samples after subtracting the diamagnetic

and

paramagnetic

background

shown

in

(a)

in

the

range

of

4000 ≤ H [Oe] ≤ 14000 , respectively.

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COC

GO

COOH

C-C ・C-H (Basal Plane) ・C-vacancy COH

C=O

C=C

1h

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3h

6h

40h 292 292

290

290

288 286 284 284 288 286 Binding Energy (eV)

282

282

Figure 3. (a) C1s XPS spectra of as-synthesized GO, and GO reduced for 1h, 3h, 6h, and 40h.

25



Figure 4. (a) Low- and (b) magnification TEM images of rGO photoreduced for 40h

C-O-C C-OH

Intensity (a.u.)

GO C=C

COOH

Lactone

C-H, C-C

rGO-40h

200

150

100

50

0

ppm

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

rGO-40h

rGO-40h

sp3 C-H GO

OH

OH C=O COO,C=C,H2O C-O-C

3000

2000

Wavenumber Figure 5. (a)

13

C-OH

1000

sp2 C-H

3000 2500

(cm-1)

C SSNMR spectra and (b) FT-IR spectra of as-synthesized GO and rGO

photoreduced for 40 h.

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Figure 6. Structures of (a) hexagonal-shaped graphene sheet with hydrogenated zig-zag edges ((C4H)24, GH0) and that with a vacancy of a carbon atom (GVH0). The carbon atom at adsorption site of hydrogen atom is labeled 1 on GH0. The vacancy on GVH0 is located at the site 1 on GH0. Labels a and b on the carbon atoms at the pore edge indicate the termination sites by hydrogen atoms. In the singly, doubly, and triply hydrogenated structures (GVH1-3), atom a, atoms b, and atoms a and b are terminated, respectively. Energy levels of (c) GH0, (d) GH1, and (e) GVH0. Spin-up and spin-down states are indicated with solid lines and broken lines, and the highest occupied π states, the lowest unoccupied π states, occupied mid-gap states and unoccupied mid-gap states are labeled H, L, Mo and Mu, respectively.

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Figure 7. Optimized structures of GH1 and GVH0-3 and corresponding Isosurface plots of the spin density. Spin-up charges and spin-down charges are indicated in yellow and light blue, respectively, and the isosurface level is set to 0.001e for both charges.

Figure 8. (a) Simulated absorption spectra of GH0-1 and GVH0-3. Note that average of extinction coefficients κxx and κyy is employed as the longitudinal axis to exhibit their typical absorption features. Side peaks emerging on the lower energy side of the main peaks are indicated with arrows.

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Figure 9. (a) PL emission and excitation map of as-synthesized GO, and GO reduced for 1 h, 3 h, and 6h; Intense PL lines marked with * around 950-1000nm are due to scattering of second order diffraction of excitation light. (b) PL mission spectra of GO and rGO reduced for 1 h, 3 h, and 6h excited at 550nm. (c) Time constants τ1 and τ2 obtained by TCSPC measurements of as-prepared GO, and GO reduced by photo-irradiation for 1 h, 3 h, and 6 h. in water at λexc = 800 nm and λem at 950 nm.

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Table and caption Table 1. Energy difference between spin-polarized and spin-unpolarized structures and magnetic moments in the spin-polarized structures.

Structures

∆E (eV)

GH1 GVH0 GVH1

−0.040 −0.042 −0.028

GVH2

−0.275 −0.046

GVH3

Magnetic moment (µB) 1.0000 0.0000 1.0000 2.0000 1.0000

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TOC

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Correlated Optical and Magnetic Properties in Photoreduced

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