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Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy.

Wang YF, Singh SB, Limaye MV, Shao YC, Hsieh SH, Chen LY, Hsueh HC, Wang HT, Chiou JW, Yeh YC, Chen CW, Chen CH, Ray SC, Wang J, Pong WF, Takagi Y, Ohigashi T, Yokoyama T, Kosugi N - Sci Rep (2015)

Bottom Line: Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature.The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO.Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Tamkang University, Tamsui 251, Taiwan.

ABSTRACT
This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature. Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

No MeSH data available.


Related in: MedlinePlus

(a,b) C and O K-edge shows XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of C (O) K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panels present C and O K-edge XMCD spectra of GO.
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f4: (a,b) C and O K-edge shows XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of C (O) K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panels present C and O K-edge XMCD spectra of GO.

Mentions: To verify that the C 2p(σ*)-derived states involve defects, which play an important role in the magnetism of GO, Fig. 4(a) displays the C K-edge XANES spectra of GO, with the photo-helicity of incident X-ray parallel (μ+) and anti-parallel (μ−) to the direction of magnetization of GO, in an applied magnetic field of ± 1 T (in the opposite direction). As stated above, the C K-edge XANES features in Fig. 4(a) within the regions 284–290 eV and 290–300 eV are known to be associated with the C 1s → 2p(π*) and 1s → 2p(σ*) transitions, respectively. Notably, the general line-shapes in the C K-edge XANES spectra of GO, presented in Fig. 4(a), differ from those of the features in the C K-edge STXM-XANES spectra of GO in Fig. 3(a,b). Apparently, the intensity of feature π* exceeds that of feature σ* in Fig. 4(a), whereas Fig. 3(a,b) present a weak feature π* and a broad and strong feature σ*. This difference arises from the fact that the C K-edge XANES spectra of GO are highly sensitive to the angle of incident light, as are those of HOPG spectrum6061. In this work, the angle of incidence θ of the X-ray between the surface normal and incident light is approximately 700 and 00 for C K-edge XANES [Fig. 4(a)] and STXM-XANES [Fig. 3(a,b)], respectively. Notably, the faint but wide-ranging features at ~280–284 eV in Fig. 4(a), may have arisen from the contamination by carbon of the grating optics at the beamline62, but, it is generally regarded as contributing equal to the photo-helicity of μ+ and μ−, so the effects of contamination by carbon can be considered to cancel each other out and not to affect the results of the XMCD analysis. The inset in Fig. 4(a) magnifies the C K-edge XANES spectra (in the region 286–298 eV), with the photo-helicity μ+ and μ− to the direction of magnetization of GO. The lower panel in Fig. 4(a) displays the C K-edge XMCD spectra, (μ− − μ+)/(μ+ + μ−). A weak but confirmed magnetic moment is associated with the C 2p states in GO at room temperature. Importantly, the intensity of the XMCD features is in the range 284–300 eV at the C K-edge of GO, attributed to both C 2p(π*)- (green, within the region 284–290 eV) and 2p(σ*)-derived states (blue, within the region 290–300 eV). Clearly, the 2p(σ*)-derived states are the main contributors to the intensity of XMCD, and the ratio of the integrated blue and green features, presented in the lower panel of Fig. 4(a), is approximately 2.9, indicating C 2p(σ*)-derived states are mostly responsible for the magnetism of GO. Still, as presented in the lower panel of Fig. 4(a), the ferromagnetic behavior of GO also involves the C 2p(π*)-derived states (green region) bound to oxygen-containing and hydroxyl groups that are observed from the C K-edge XMCD results.


Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy.

Wang YF, Singh SB, Limaye MV, Shao YC, Hsieh SH, Chen LY, Hsueh HC, Wang HT, Chiou JW, Yeh YC, Chen CW, Chen CH, Ray SC, Wang J, Pong WF, Takagi Y, Ohigashi T, Yokoyama T, Kosugi N - Sci Rep (2015)

(a,b) C and O K-edge shows XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of C (O) K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panels present C and O K-edge XMCD spectra of GO.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4612711&req=5

f4: (a,b) C and O K-edge shows XANES spectra of GO with photo-helicity of incident X-rays parallel (μ+) and anti-parallel (μ−) to direction of magnetization, respectively. Inset magnifies π-σ (σ) region of C (O) K-edge XANES spectra with incident X-rays μ+ and μ− to direction of magnetization. Bottom panels present C and O K-edge XMCD spectra of GO.
Mentions: To verify that the C 2p(σ*)-derived states involve defects, which play an important role in the magnetism of GO, Fig. 4(a) displays the C K-edge XANES spectra of GO, with the photo-helicity of incident X-ray parallel (μ+) and anti-parallel (μ−) to the direction of magnetization of GO, in an applied magnetic field of ± 1 T (in the opposite direction). As stated above, the C K-edge XANES features in Fig. 4(a) within the regions 284–290 eV and 290–300 eV are known to be associated with the C 1s → 2p(π*) and 1s → 2p(σ*) transitions, respectively. Notably, the general line-shapes in the C K-edge XANES spectra of GO, presented in Fig. 4(a), differ from those of the features in the C K-edge STXM-XANES spectra of GO in Fig. 3(a,b). Apparently, the intensity of feature π* exceeds that of feature σ* in Fig. 4(a), whereas Fig. 3(a,b) present a weak feature π* and a broad and strong feature σ*. This difference arises from the fact that the C K-edge XANES spectra of GO are highly sensitive to the angle of incident light, as are those of HOPG spectrum6061. In this work, the angle of incidence θ of the X-ray between the surface normal and incident light is approximately 700 and 00 for C K-edge XANES [Fig. 4(a)] and STXM-XANES [Fig. 3(a,b)], respectively. Notably, the faint but wide-ranging features at ~280–284 eV in Fig. 4(a), may have arisen from the contamination by carbon of the grating optics at the beamline62, but, it is generally regarded as contributing equal to the photo-helicity of μ+ and μ−, so the effects of contamination by carbon can be considered to cancel each other out and not to affect the results of the XMCD analysis. The inset in Fig. 4(a) magnifies the C K-edge XANES spectra (in the region 286–298 eV), with the photo-helicity μ+ and μ− to the direction of magnetization of GO. The lower panel in Fig. 4(a) displays the C K-edge XMCD spectra, (μ− − μ+)/(μ+ + μ−). A weak but confirmed magnetic moment is associated with the C 2p states in GO at room temperature. Importantly, the intensity of the XMCD features is in the range 284–300 eV at the C K-edge of GO, attributed to both C 2p(π*)- (green, within the region 284–290 eV) and 2p(σ*)-derived states (blue, within the region 290–300 eV). Clearly, the 2p(σ*)-derived states are the main contributors to the intensity of XMCD, and the ratio of the integrated blue and green features, presented in the lower panel of Fig. 4(a), is approximately 2.9, indicating C 2p(σ*)-derived states are mostly responsible for the magnetism of GO. Still, as presented in the lower panel of Fig. 4(a), the ferromagnetic behavior of GO also involves the C 2p(π*)-derived states (green region) bound to oxygen-containing and hydroxyl groups that are observed from the C K-edge XMCD results.

Bottom Line: Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature.The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO.Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics, Tamkang University, Tamsui 251, Taiwan.

ABSTRACT
This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature. Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

No MeSH data available.


Related in: MedlinePlus