Origins of sp3C peaks in C1s X-ray Photoelectron Spectra of Carbon

Letter pubs.acs.org/ac

Origins of sp3C peaks in C1s X‑ray Photoelectron Spectra of Carbon Materials Ayaka Fujimoto,† Yasuhiro Yamada,*,† Michio Koinuma,‡ and Satoshi Sato† †

Graduate School of Engineering, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Graduate School of Science and Technology, Kumamoto University, 2-29-1 Kurokami, Kumamoto, 860-8555, Japan

‡

S Supporting Information *

ABSTRACT: X-ray photoelectron spectroscopy (XPS) is among the most powerful techniques to analyze defective structures of carbon materials such as graphene and activated carbon. However, reported assignments of defects, especially sp3C and sp2C, are questionable. Most reports assign sp3C peaks to be higher than sp2C peaks, whereas a few reports assign sp3C peaks to be lower than sp2C peaks. Our group previously reported that calculated binding energies of sp3C were basically lower than those of sp2C. This work clarified that one of the reasons for the prevailing ambiguous assignments of sp3C peaks is charging effects of diamond.

C

arbon materials such as graphene, carbon nanotube, carbon fibers, and activated carbon materials have been intensively studied especially in the past few decades.1−3 Many researchers have analyzed carbon materials using various techniques to clarify the difference of those structures at atomic scale.4−7 Especially, X-ray photoelectron spectroscopy (XPS) has been well-known to be among the most powerful tools to determine the structures of carbon materials,8−38 but the structures are extremely complicated because carbon materials contain various defects such as edges, functional groups, pentagons, heptagons, and sp3C.39 Thus, assignments of C1s spectra are still under debate. The history of XPS analysis for carbon materials with sp3C and sp2C has been summarized by Chu and Li.9 Lascovich and Scaglione differentiated the peaks of sp2C and sp3C in 1994.10 Jackson and Nuzzo used this assignment to determine the areal ratio of sp2C and sp3C in 1995.11 Diaz et al. proposed a method to clarify the difference between sp2C and sp3C based on the difference of C1s shifts between diamond and graphite in 1996.12 Similarly, a number of studies have used the difference of C1s shift between sp2C and sp3C for their assignments.13,16,17,19−32 All of these studies assigned the peak for sp3C in C1s spectra as being higher than that for sp2C (Figure 1a and Table S1). The results seem to conclude that the binding energy of the C1s peak for sp3C is higher than that of sp2C without any question. Contrary to the prevailing assignments of sp3C for C1s spectra, the assignments of only a few research groups were completely opposite.14,15,37,38 They indicated that the peak position of sp3C is lower than that of sp2C (Figure 1b and Table S1). We have recently demonstrated that the peak position of sp3C is basically lower than that of sp2C using a © 2016 American Chemical Society

Figure 1. Reported peak positions of sp3C versus sp2C for C1s XPS analysis. (a) Binding energy for the peak originated from sp3C is higher than that from sp2C. (b) Binding energy for the peak originated from sp3C is lower than that from sp2C. (c) Plots of reported binding energies of the peak originated from sp3C relative to reported binding energies of the peak originated from sp2C.11−30,37,38 Horizontal axis is binding energy of sp3C, and the longitudinal axis is the difference of binding energy between sp3C and sp2C. Circle: pure diamond and graphite. Triangle: other carbon materials.

calculation,32 which agrees with the results of those few groups. Our calculation method has already been proven to be close to most of the experimental values,32−36 but we have not shown clear evidence of the calculated results. It is known that charging effects largely influence the peak top of C1s spectra. Received: April 5, 2016 Accepted: June 4, 2016 Published: June 4, 2016 6110

DOI: 10.1021/acs.analchem.6b01327 Anal. Chem. 2016, 88, 6110−6114

Letter

Analytical Chemistry Insulating diamond has been reported to show a wide range of peaks from −1.5 to 3 eV shifted from sp2C (Figure 1c and Table S1).11−30,37,38 Thus, charging effects rather than the presence of sp3C could be one of the actual origins for the conventionally assigned sp3C peaks of C1s spectra. From the point of view of analyses of general carbon materials with various defects, the increment of full width at half-maximum (fwhm) of C1s spectra of carbon materials compared to graphite (sp2C−sp2C) could be caused by the presence of electronwithdrawing functional groups, C−H groups, pentagons, and heptagons in graphene.32,33,35 These defects can also have other origins of reported sp3C peaks of carbon materials. In this work, we investigated the effect of charging of diamond on highly oriented pyrolytic graphite (HOPG) as well as adapalene (Figure S7a), which includes sp3C (adamantane) bonded with sp2C (benzene and naphthalene), using mainly actual XPS analyses and calculated XPS spectra. Monocrystalline diamond powder (1 μm, Sigma-Aldrich Co.) was heated at 623 K for 2 h under hydrogen gas to eliminate thermally unstable functional groups. The temperature was selected to avoid changing the structure of diamond. Diamond powder was analyzed by Raman spectroscopy (NRS2100, JASCO, excitation wavelength of laser; 532 nm) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy (FT-IR-4200, JASCO Corp.), and the purification of diamond was confirmed (Figures S1 and S2). The reduced diamond powder was dispersed in 2-propanol by sonication, and 3, 15, and 45 g L−1 of diamond-dispersed solution were spread well over HOPG (ZYH, NT-MDT Co.) using spin coating at 1000 rpm. After the solvent was dried, diamond on HOPG was sandwiched with another HOPG by being compressed at 5 MPa to embed diamond on HOPG. These samples are named as 3MD, 15 MD, and 45 MD. The state of diamond on HOPG was analyzed by scanning electron microscopy (SEM). These embedded samples and adapalene (C28H28O3, Figure S7a, 98.0%, Tokyo Chemical Industry Co., Ltd.) were analyzed using XPS (Sigma Probe, Thermo Fisher Scientific Inc.). Adapalene was selected to show a peak originating from sp3C directly bonded with sp2C. Line shapes of spectra were Gaussian type for diamond and an asymmetric Voigt type35 for HOPG. The ratio of Lorentzian function to Gaussian function (m) in Voigt function was 0.87. The asymmetry factor (a) was 0.27. The Voigt function was used because of better fitting than the Doniach-Šunjić type.35 Density functional theory calculation was conducted using B3LYP/6-31g(d) with Gaussian 09.40 Modeled structures were optimized and XPS spectra were simulated using the keyword of opt pop = full gfprint. Charge and spin multiplicity were set as 0 and 1, respectively. States of diamond on HOPG (Figure 2a−d) can be divided into 6 (Figure 2e). These states were utilized to assign peaks for C1s spectra (Figure 3 and Tables 1) and to count the number of diamonds in SEM images (Table 2). Peaks 1 and 2 in Figure 2e are suggested to correspond to HOPG and diamond embedded into HOPG, respectively. Peaks 3 and 4 are suggested to correspond to diamond embedded slightly into HOPG and diamond just touching on HOPG, respectively. Peaks 5 and 6 are suggested to correspond to diamond on top of diamond embedded in HOPG and diamond on top of diamond just touching HOPG, respectively. Figure 3 shows C1s spectra of diamond on HOPG at different diamond concentrations. The peak top of C1s spectra of HOPG was 283.8 eV after being adjusted at Fermi level as 0 eV (Figure

Figure 2. SEM images and a schematic side view of diamond embedded in HOPG. (a) 3MD. (b) A magnified image of the rectangle in (a). (c) 15MD. (d) 45MD. (e) A schematic side view of diamond on HOPG. The number of peaks in (e) corresponds to the number of peaks in Figure 3.

Figure 3. C1s XPS spectra of diamond embedded in HOPG. (a) HOPG. (b) 3MD. (c) 15MD. (d) 45MD. (i) Original spectra. (ii) Magnified spectra. The number of peaks corresponds to the number in Figure 2e and Table 1.

S3). The peak tops of C1s spectra of each sample were adjusted to 283.8 eV. The intensity of the spectrum of HOPG was subtracted from spectra of each sample. The difference spectra were utilized for assignments of peaks of diamond on HOPG. At 3MD in Figure 3, peaks 2 and 3 appeared at 284.1 and 285.5 eV, respectively. It is obvious that these peaks originate from the presence of diamond, but especially peak 2 at 284.1 eV, which is +0.3 eV relative to the peak of sp2C, was much 6111

DOI: 10.1021/acs.analchem.6b01327 Anal. Chem. 2016, 88, 6110−6114

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Analytical Chemistry Table 1. Binding Energies of sp2C−sp2C (Peak 1) and sp3C−sp3C (Peaks 2−6) Used in Figure 3a

a

samples

peak 6

peak 5

peak 4

peak 3

peak 2

HOPG 3MD 15MD 45MD

− − − 289.0 (1.9)

− − 286.9 (1.5) 287.3 (1.5)

− − 286.0 (1.6) 285.9 (1.6)

− 285.5 (1.2) 285.5 (1.2) 285.3 (1.2)

− 284.1 (1.1) 284.1 (1.1) 283.9 (1.1)

peak 1 283.8 283.8 283.8 283.8

(0.8) (0.8) (0.8) (0.8)

The numbers in parentheses indicate FWHMs of each peak.

because of a difference in the analytical condition compared to the method conducted by Mezzi and Kaciulis. The difference of states can also be distinguished from the color of diamond in SEM images (Figure 2a−d), because the color of charged diamond becomes whiter than that of less charged diamond. From 10 SEM images of diamond on HOPG, the number of diamonds were counted and the states of diamond on HOPG were divided into 3 states (Table 2). As the concentration of diamond increased from 3MD to 15 MD, the percentage of charged diamond increased. From the results of SEM, XAES, and XPS (Table 3), the procedure of peak fitting of C1s XPS spectra in Table 2 and Figure 3 is acceptable.

Table 2. Counts of Diamond on HOPG in SEM Images counts and percentage of diamond particles on HOPG sample name

peak 2a

peaks 3 and 4a

peaks 5 and 6a

3MD 15MD 45MD

193 (64%) 984 (42%) 1228 (31%)

96 (32%) 807 (34%) 1655 (42%)

12 (4%) 564 (24%) 1076 (27%)

a The number of peaks corresponds to the number of peaks in Figure 2e.

lower than most of the reported peaks for sp3C (Figure 1 and Table S1). The peak position of peak 2 was closer to that of HOPG (Figure 3), indicating that peak 2 corresponded to the peak of sp3C with the least charging effect being close to the peak of sp2C. As the concentration of diamonds increases from 3MD to 45MD, diamonds overlap with other diamonds (Figure 2a−d) and new peaks 4, 5, and 6 appeared (Figure 3) because of charging effects. C1s spectra of HOPG have been well studied by Leiro et al.;41 the peaks between 288 and 291 eV are known to be satellite peaks, but the increment of peak 6 indicates that charged diamond increased peak 6. These results clearly show that peak 2 is the closest to the actual peak for diamond without charging effects. These results also show that most of the reported assignments of sp3C, which are ca. + 0.8 eV or even as high as 3 eV (Figure 1), are the peak of charged sp3C or possibly different defects in carbon materials. Figure 4 shows D parameters of samples analyzed by X-ray Auger electron spectroscopy (XAES) under the same

Table 3. Percentages of sp2C Obtained by Various Methods sp2C/(sp2C + sp3C)/% sample name

SEM

HOPG 3MD 15MD 45MD

100 91 76 54

XAES 89 85 75 56

(100)a (96)a (84)a (63)a

C1s XPS 96 91 80 52

(100)a (95)a (83)a (54)a

a

The numbers in parentheses were obtained by adjusting 89% for XAES and 96% for C1s XPS to 100%.

The result of actual XPS spectra can also be explained by the result of simulated C1s XPS spectra. The difference of peak tops between sp3C and sp2C in actual spectra was +0.3 eV, but from the calculated results in Figure 6, the difference of peak tops between sp3C and sp2C was ca. −0.6 eV as results of simulated C1s XPS spectra using model compounds in Figure 5. It indicates that +0.3 eV can be further decreased and possibly become negative values by complete removal of charging effects. We also analyzed C1s XPS spectra of adapalene with both sp2C and sp3C experimentally and theoretically (Figure S7). This result also exhibited that the peak position of sp3C is lower than that of sp2C, indicating that peak positions largely influenced by charging effects and the assignment of sp3C in many reports could be inappropriate depending on the analysis conditions and types of defects in carbon materials. In conclusion, one of the origins for ambiguous assignments of sp3C of C1s XPS spectra for carbon materials is charging effect, which is caused by the disconnection of the pathway of electrons between sp2C and sp3C. Another origin for the increment of fwhm of C1s spectra is the presence of defects such as pentagon, heptagons, and functional groups as explained in other papers reported by our groups.32,33 Thus, it is essential to consider these two factors to assign the XPS spectra in detail.

Figure 4. First derivative of CKLL peaks analyzed by XAES. (a) HOPG. (b) 3MD. (c) 15MD. (d) 45MD. D parameters are written in the middle of each spectrum.

conditions as C1s spectra. The original CKLL spectra were shown in Figure S3. Mezzi and Kaciulis have reported that the ratio of sp2C/sp3C can be determined by the D parameters, which are differences between maximum and minimum binding energies of the first derivative of CKLL spectra.18 The higher the D parameter is, the higher the percentage of sp2C is. D parameters obtained from Figure 4 were converted into the percentage of sp2C (Table 3). The percentage of sp2C decreased from 89% to 56%, as the concentration of diamonds on HOPG increased. The reason for 89% of HOPG is probably 6112

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Analytical Chemistry

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Figure 5. Structures with sp2C−sp3C simulated in Figure 6. (a) C26H30. (b) C35H36. (c) C44H42. (d) C53H48. (e) C68H56.

Figure 6. Simulated C1s XPS spectra of structures shown in Figure 5. Peak tops of sp2C were fixed at 283.80 eV. sp2C−H and sp3C−H were removed from the spectra in this figure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b01327. Summary table for reported binding energies of sp2C and sp3C, Raman and IR spectra, XPS spectra at Fermi level, actual O1s XPS spectra of diamond on HOPG, CKLL XAES, and actual and calculated C1s XPS spectra of adapalene (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-43-290-3376. E-mail: [email protected]. jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ichijyu Industrial Science and Technology Promotion Foundation in Japan. REFERENCES

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DOI: 10.1021/acs.analchem.6b01327 Anal. Chem. 2016, 88, 6110−6114