Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy

SAA-117904; No of Pages 8 Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx

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Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy Yong Zhou, Juan Lin, Xue-Ying Nie, Xiu-Neng Song ⁎, Yong Ma ⁎, Chuan-Kui Wang School of Physics and Electronics, Shandong Normal University, Jinan, Shandong 250014, People's Republic of China

a r t i c l e

i n f o

Article history: Received 27 September 2019 Received in revised form 27 November 2019 Accepted 2 December 2019 Available online xxxx Keywords: C20@C60 isomers X-ray photoelectron spectra Near-edge X-ray absorption fine structure spectra

a b s t r a c t The C 1s X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of two buckyonion fullerene C20@C60 isomers were theoretically simulated by density functional theory (DFT) method. In this paper, we mainly investigated the relationship between spectroscopy and structure and obtained the spectral dependence on the structures with different symmetries. Our results showed that both XPS and NEXAFS spectra exhibited remarkable dependence on the molecular structures, thus these two spectroscopic techniques could be adopted to identify the two C20@C60 isomers effectively. Additionally, we found that the building block approach could not predict the spectra for C20@C60 isomers due to the strong interaction between the two layer cages. Finally, we studied the decompositions of the total spectra from carbon atoms in different local environments to analyse the origins of the main features. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of the buckminsterfullerene C60 by Smalley in 1985 [1], nanometric carbon clusters have received a great deal attention from experimentalists and theorists for the purpose of designing and developing functional materials with their unique physical and chemical properties. One representative discovery of the carbon based nanomaterials is carbon nano-onions (CNOs, also named multishell fullerenes, buckyonions or hyperfullerenes) with a nearly spherical shape consisting of several concentric enclosed fullerene-like carbon shells in a “Russian doll” manner [2–5]. Due to the large external surface area and the high electrical conductivity as well as the possibility for largescale synthesis [6], CNOs have become the key point for many electrochemical energy storage (EES) [7] applications such as electrode materials in supercapacitors [8] and anode materials in lithium-ion batteries [9,10] in the recent years. Besides, CNOs have also been proposed in other application fields, ranging from biomedical applications like biological imaging [11,12] and biological sensing [13] to optical limiting [14] and molecular junctions in STM [15]. More scientifically intriguing, the multishell fullerenes were regarded as possible candidates to explain the intense UV absorption feature appearing at 217.5 nm of the interstellar medium spectra, which was the initial motivation for Smalley experiments in 1985 [2,16]. The first synthesis of CNOs in the laboratory was accomplished by Ugarte under the intense electron beam ⁎ Corresponding authors. E-mail addresses: [email protected] (X.-N. Song), [email protected] (Y. Ma).

irradiation of carbon soot with a transmission electron microscope in 1992 [17]. Since then, other methods were developed to enlarge the production scale, such as high temperature annealing of nanoparticles [18], energetic carbon ion implantation on silver particles [19], the flame synthesis which is carried out through a more sooty heptane flames at a stagnation-point liquid-pool system [20], and laser vaporization method [21] with the yield of mixed multi-shell fullerenes up to 90%. At the same time, the CNOs were characterized experimentally by a variety of physical methods to further study their properties and applications. For example, the morphology and microstructure of the purified CNOs were observed by the field-emission scanning electron microscope (SEM) and the transmission electron microscope (TEM), and their surface chemistries were investigated by X-ray photo emission spectroscopy (XPS) and Raman spectroscopy; besides, their phase compositions were researched by X-ray diffraction (XRD) [22]. However, large amounts of multishell fullerene isomers make the relevant research become more complicated. Thus isomer identification becomes one of the most fundamental points in the investigations of the multishell fullerenes. In the previous researches of the isomer identification of single layer fullerenes, soft X-ray spectroscopies which are based on the specificities and sensitivities of the atoms in different local chemical environments have been widely employed to explore the electronic structures and chemical constructions of the molecules by core-level excitations or de-excitations. Specifically, the X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been successfully employed for the isomer identifications of small fullerene C34 [23], heterofullerenes

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C48N12 [24], C58N2 [25] and C58B2 [26], large fullerenes C76 [27], C78 [28], C82 [29], C56 [30], C66 [31] and C72 [32] as well as two Ih-symmetrybreaking C60 [33] which could be stabilized by exohedral chlorination isomers. XPS mainly depicts the ionization process of core electrons and provides the information of core orbitals. While NEXAFS spectroscopy corresponding to the excitation process of the core electrons from the core level to virtual orbitals comprehensively describes the features of the unoccupied orbitals. These soft X-ray spectroscopies could also be adopted to characterize the various isomers of multishell fullerenes [34] and here we focus on the theoretical identification for simplest potential double-shell fullerene C20@C60. C20@C60, first proposed by Lemi Türker [35], is constituted by the smallest fullerene C20 [36] and the fullerene C60 that has been profoundly investigated both experimentally and theoretically. Despite many difficulties encountered in isolating the pure samples, the theoretical simulations of C20@C60 fullerene have obtained a series of achievements. The geometries, energies, relative stabilities, electronic properties, and inter-shell interactions of C20@C60 isomers have been investigated in theory at the semi-empirical and DFT levels in previous literatures [37]. And the photo absorption spectrum of C20@C60 has been calculated by means of TDDFT recently, which found that the spectrum of the composite system differs significantly from the sum of spectra of the two isolated fullerenes C20 and C60 due to strong geometrical distortion of the system [38]. A further study of the electronic and spectroscopic properties of the simplest buckyonion fullerene would be helpful to gain a deeper comprehension of such fullerene systems. In the current work, the geometrical and electronic structures of the two C20@C60 isomers and a single layer fullerene C80 (with Ih symmetry, as a contrast to the double-shell fullerenes) have been simulated with DFT at a higher level than the previous research based on the firstprinciples calculations. Then, we calculated the carbon K-shell (1 s) XPS and NEXAFS spectroscopy using gradient corrected density functional theory to characterize the C20@C60 isomers. Furthermore, the sum of XPS and NEXAFS spectra of the two isolated fullerenes C20 and C60 were calculated through the building block approach by which the total spectrum of a large complex system could approximately equal to a simply linear superposition of the spectra of all constituent functional groups to reflect the changes of electronic structures after encapsulation. The aim of present work is to study the relation between structure and spectroscopy and inspect the isomer dependence of spectroscopy on the local environment, as well as explore the influences on the spectra due to strong inter-shell interaction in the C20@C60. We hope that the theoretical study of the C20@C60 isomers by soft X-ray spectroscopies could shed some light on the isomer identifications and characterizations of the buckyonion fullerenes in further experimental and theoretical studies. 2. Computational details At first, three-dimensional models of the isolated fullerenes C20 (C2h), C60 (Ih) and two C20@C60 isomers, as well as free fullerene Ih-C80 were simulated by using GaussView [39] in our study. Then the geometries of the above-mentioned CNOs and fullerenes were optimized with DFT at B3LYP/6-31G (d, p) [40,41] level applying the Gaussian09 program package [42]. On the basis of the optimized structures, we calculated the C 1s XPS and NEXAFS spectra of the studied CNOs and fullerenes by StoBe program [43] at the DFT level using the gradient corrected Becke (BE88) exchange functional [44] and Perdew (PD86) correlation functional [45]. And the oscillator strength obtained through the exchange-correlation functionals has been proved to be coincident with experiments in previous research [28,46,47]. We adopted the triple-ς quality individual gauge for localized orbital (IGLO-III) basis set [48] for the description of core-excited carbon atoms and the triple-ς plus valence polarization (TZVP) basis set for other atoms. Besides, in an effort to facilitate the self-consistent field (SCF) convergence of the core-hole state, miscellaneous auxiliary basis sets were set for all

atoms [43], and specially, model core potentials were added to depict the non-excited carbon atoms. In addition, we adopted a doublebasis-set method for the excited carbon atoms, in which a normal molecule orbital basis set was used to minimize the energy and an additional augmented diffuse basis set (19s, 19p, 19d) [49] was used to generate the transition moments and the excitation energies. More concretely, the X-ray photoelectron spectroscopy was relevant to the ionization potential (IP) values of atoms. Hence, we adopted the Kohn-Sham (KS) scheme for the calculation of the C 1 s ionization potential [50,51], IP i ¼

N−1

EFCH − N EGS

ð1Þ

Here N−1EFCH represented the energy of the fully optimized coreionized state (full core hole state) and NEGS represented the groundstate energy. This ΔKS scheme could give the accurate ionization energies because the relaxations caused by the introduced core-hole were fully considered in the calculation procedure. With respect to the Xray absorption spectroscopy, two states, namely the ground state (initial state) and the core-excited state (final state), were involved in this process. As the final-state rule [52–54] stated, accurate absorption spectra could be gotten only by the final-state wave function in the finite molecular systems. Considering the process of transition was much faster than the relaxation of other passive electrons, we could neglect the relaxation and simplify this problem into a single-electron picture depicted by a pair of molecular orbitals (MOs) of the final state. Moreover, in the calculation of the NEXAFS spectra, we adopted the full core hole (FCH) approximation, in which the core-ionized state as a reference state represented the core-excited state. For the transition from initial state to final state, the absorption oscillator strength was given as the following formula [55], f if ¼

 2mε if  jbψ f jxjψi N j2 þ jbψ f jyjψi N j2 þ jbψ f jzjψi N j2 2 3ℏ

ð2Þ

where ψi and ψf were the core and unoccupied MOs involved in the transition and εif denoted the corresponding transition energy. Considering the random orientation of molecules in reality, the absorption oscillator strength was calculated by averaging over the x, y, z components. Then, for the purpose to calculate the absolute energy positions of the peaks, the absorption spectra obtained by the FCH approach were calibrated through calculating the transition energy from C 1s to the lowest unoccupied molecular orbital (LUMO) with the ΔKS scheme [50,51], εΔKS 1s→LUMO ¼

N

E1s→LUMO − N EGS

ð3Þ

Here NE1s→LUMO denoted the energy of the core-excited state with one core electron excited to LUMO and NEGS denoted the GS energy. After energy calibration, the first spectral feature that represented the transition from 1 s to the LUMO was consistent with the accurate excitation energy of the same transition calculated by the scheme. Finally, we added a uniform correction of 0.2 eV to the calculated IP's or transition energies considering the relativistic effects associated with the introduced core-hole [51]. Actually, both the XPS and NEXAFS spectra were finally continuous spectra obtained by broadening the stick spectra calculated above. In this work, after calculating the IP's of the symmetry independent carbon atoms, the XPS spectra were engendered through broadening the obtained IP values by a Gaussian function with full width at half-maximum (fwhm) of 0.2 eV. As for NEXAFS spectra, a Gaussian function with fwhm of 0.4 was applied to convolute the oscillator strengths under the IP, while in the continuum region above the IP, the Stielties imaging approach [56] was adopted to form the spectra. For all the studied molecules, we calculated the spectra of the symmetry-independent (nonequivalent) carbon atoms at first, and then summed up the contribution of each nonequivalent carbon atom according to their relative abundance to obtain the total spectra.

Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904

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Similarly, the sum of XPS and NEXAFS spectra of the two isolated fullerenes C20 and C60 were obtained by summing up the contribution of nonequivalent carbon atom belonged to the C20 and C60 fullerenes. 3. Results and discussions 3.1. Structures Fullerenes C20, C60, and Ih-C80 were firstly optimized, of which the C20 and C60 were used to construct the C20@C60, and the Ih-C80 was used as a contrast. Actually, only two highly symmetric C20@C60 isomers, the D5d-C20@C60 and the Ih-C20@C60, can exist because the interlayer distance in C20@C60 is quite short which causes the C20 to be confined [37]. Optimized structures of the above molecules are displayed in Fig. 1. As described in previous research, the D5d-C20@C60 engenders slightly geometrical distortion upon encapsulation and shows an elongated fusiform symmetry while the Ih-C20@C60 keeps the spherical structure. As shown in Fig. 1, in the D5d-C20@C60, there are two pentagons of the C20 facing the pentagons of the C60 in the same orientation. The nearest distance between the inner and outer layers is only 1.524 Å, which is comparable with the length of C\\C

3

bond in the systems, so the inter-shell C\\C bonds can be existed. Whereas in the Ih-C20@C60, each pentagon of C20 is opposite to the corresponding pentagon in the C60. And the distance of the inner atoms and outer atoms is larger, so there're no inter-shell C\\C bonds in Ih-C20@ C60. Furthermore, after calculating Mulliken atomic charge of the two isomers, we can find that the electron transfer from the atoms of C60 to that of C20 in Ih-C20@C60. In D5d isomer, the atoms of C60 shell that are close to the inner atoms can be the electron acceptor while other atoms are the electron donor. Table 1 lists the average radii of the free C20 and the free C60 as well as encapsulated C20 and C60 in the two C20@C60 isomers. We should note that C60 and the Ih-C20@C60 are spherical which means each C atom of them is located on the surface of a sphere, but geometries of C20 and the D5d-C20@C60 are not standard spherical, instead, they are polyhedral. Therefore, the radii of the latter two carbon cages in the Table 1 are the average distances between the carbon atoms and the center of mass [57]. As displayed in Table 1, by comparing the radii of free fullerenes and the inner/outer shell of the double-shell fullerenes, it's evident to see that the inner fullerene is shrunk while the outer fullerene is expended upon encapsulation. (This conclusion is in good accordance with the previous results). And the overlap of electronic clouds which

Fig. 1. (a) Optimized molecular structures of three single-wall fullerenes C20, C60 and IhC80. (b) Optimized molecular geometries of two C20@C60 isomers in different perspectives.

Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904

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Y. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx Table 1 Average radii of free fullerenes C20 and C60 as well as the C20 shell and C60 shell of two C20@C60 isomers. Molecule

Radius

Free C20 Free C60 C20 part of D5d-C20@C60 C60 part of D5d-C20@C60 C20 part of Ih-C20@C60 C60 part of Ih-C20@C60

2.03477 3.54953 1.98692 3.79987 1.91868 3.84384

causes steric repulsion owing to the Pauli exclusion principle should be responsible for this phenomenon [57]. The total/relative energy of per atom and the energy gaps between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the two C20@C60 isomers and three free fullerenes Ih-C80, C20 and C60 are displayed in Table 2. The relative energy of per atom of D5d-C20@C60 is 21.958 kcal/mol, which is lower than that located at Ih-C20@C60 whose relative energy is 33.233 kcal/mol, and the corresponding energy in free C20 (20.155 kcal/mol) and C60 (0.897 kcal/mol) are both lower than the two composite systems. Actually, result of that the relative energy of per atom of D5d- C20@C60 is slightly higher than that of individual fullerene C20 is different from the previous estimates in which the energy of C20 is slightly higher than D5d-C20@C60 [37]. The slightly different computational methods (B3LYP/3-21G* level was used in calculation before, but we adopt B3LYP/6-31G (d, p) at present) should be responsible for this difference. Hence, according to the relative energy of per atom, the order of the relative stability should be C60 N C20 N D5d-C20@C60 N Ih-C20@C60. Moreover, the HOMO-LUMO gap can also reflect chemical stability of molecular structures with the larger gap corresponding to higher stability. From the Table 2 we can see that the calculated HOMO-LUMO gaps follow the order of C60 N C20 N D5d-C20@C60 N Ih-C20@C60, which further validates the order of relative stability of the composite onion-like systems and their components. In addition, the reduction of the HOMOLUMO gaps of C20@C60 isomers also partially reveals the large influence of the interaction between the inner-shell and outer-shell fullerene on the structure. Although the total energy per atom of Ih C80 is the lowest, its HOMO-LUMO gap is not large, so its stability cannot compare with the above fullerenes simply according to the HOMO-LUMO gap. We also calculated relative energies and HOMOLUMO gaps of C 20 @C60 systems with some relevant functionals with the dispersion effect, including CAM-B3LYP [58], M06 [59], PBE1PBE [60] and wB97XD [61]. The calculated results are listed in Table 2. We can see that the trends of relative energies and HOMOLUMO gap calculated by different functionals are the same. This is mainly related to the small interval between the inside and outside cages. The two cages are very close to each other, which results to strong interaction between the two cages. In the research of fullerene structures by spectra, it's common to group the carbon atoms of fullerenes in terms of the local environment. Here we classify the carbon atoms of D5d- C20@C60 into six types, four types for C60 layer and two types for C20 layer:(1) the Type 1 of C60 layer, in which the carbon atoms belonging to the pentagons that face the pentagons of inner layer in the same orientation (10 atoms in

Fig. 2. The calculated C 1s XPS spectra of two C20@C60 isomers and three free fullerenes (IhC80, C20 and C60) as well as the building block of C20 and C60.

total); (2) the Type 2 of C60 layer, in which the carbon atoms connect with the carbon atoms of Type 1 (10 atoms in total); (3) the Type 3 of C60 layer, in which the other carbon atoms that the atoms of Type 2 connect with (20 atoms in total); (4) the Type 4 of C60 layer, in which the other carbon atoms that the atoms of Type 3 connect with (20 atoms in total); (5) the Type 1 of C20 layer, in which the carbon atoms belongs to the pentagons that face the pentagons of outer layer in the same orientation (10 atoms in total); (6) the Type 2 of C20 layer, in which the carbon atoms are the rest atoms in C20 (10 atoms in total). While in the Ih-C20@C60, because each pentagon of the C20 layer and C60 layer face each other in the opposite orientation, we can regard all the carbon atoms of C60 layer as the atoms belong to the Type 1 of C60 layer and all the carbon atoms of C20 layer as the atoms of the Type 1 of C20 layer, that means, all the atoms of the C60 are equivalent, all the atoms of the C20 as well. As for Ih-C80, the carbon atoms are classified into two conventional types, which are corannulene site (the carbon atom is shared by one pentagon and two hexagons and connects a hexagon through the common bond owed by the adjacent hexagons) and pyrene site (the carbon atom is shared by three hexagons). For the convenience of expression, the Type 1, 2, 3 and 4 of C60 layer in D5d-C20@C60 are respectively represented as C1, C2, C3 and C4 and the Type 1 and 2 of C20 layer as C5 and C6. In Ih-C20@C60, the Type 1 of C60 layer and the Type 1 of C20 layer are expressed as C1 and C2. And in isolated fullerene Ih-C80, we define the carbon atoms belonging to corannulene site and pyrene site as C1 and C2, separately.

Table 2 Relative energy of per atom and energy gaps between HOMO and LUMO of two C20@C60 isomers and three single-wall fullerenes (Ih-C80, C20 and C60) under different functions. Molecule

D5d-C20@C60 Ih-C20@C60 Ih-C80 C20 C60

Relative energy of per atom (kcal/mol)

HOMO-LUMO gap (eV)

B3LYP

CAM- B3LYP

M06

PBE1PBE

wB97XD

B3LYP

CAM- B3LYP

M06

PBE1PBE

wB97XD

21.9 33.2 0 20.1 0.9

32.5 45.0 0 26.2 0.2

30.4 43.0 0 24.2 0.4

32.4 38.7 0 26.1 0.5

31.5 44.0 0 25.2 0.1

1.2 0.3 0.8 1.9 2.7

2.4 2.1 2.1 2.9 5.0

1.1 0.8 0.9 1.1 3.2

1.1 0.8 0.8 1.0 3.1

3.4 2.9 3.0 4.0 6.1

Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904

Y. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx Table 3 Energy positions of the spectral features in calculated C 1s XPS spectra of the two C20@C60 isomers and Ih-C80 as well as the building block of C20 and C60. Molecule

a

b

c

d

D5d-C20@C60 Ih-C20@C60 Ih-C80 C20 + C60

289.73 289.01 289.49 289.97

290.42 290.25 289.96 290.36

291.06 – – 290.87

291.46 – – –

3.2. XPS spectra The X-ray photoelectron spectroscopy that shows the IP values of carbon atoms is able to reflect the characteristic of core electrons that are unique for each element, so it is widely adopted to analyse substance structure. The calculated C 1s XPS spectra of the two C20@C60 isomers and three free fullerenes Ih-C80, C60 and C20, as well as C20 + C60 from building block method are shown in Fig. 2. Besides, the specific energy position of every feature of the spectra is displayed in Table 3. D5dC20@C60 and Ih-C20@C60 exhibit obviously different XPS spectra: D5dC20@C60 shows four distinct features in which the first feature a at the lowest energy position arises at 289.73 eV and the highest peak b arises at 290.42 eV, while Ih-C20@C60 only show two distinct features including the first peak a at 289.01 eV and one strong peak b at 290.25 eV which are both located at the lower energy position than the corresponding features of D5d-C20@C60 spectrum. Based on the significant differences, it's easy to identify the C20@C60 isomers through the XPS spectra. IhC80 also exhibits two distinct features in the XPS spectrum; whereas, the strong peak appears at the lower energy and the weak peak arises at higher energy, which is quite contrary to the Ih-C20@C60 spectrum. Furthermore, the energy interval from feature a to feature b in Ih-C80 is around 0.47 eV (about from 289.49 eV to 289.96 eV), which is smaller than 0.69 eV (about from 289.73 eV to 290.42 eV) in D5d-C20@C60 and 1.24 eV (about from 289.01 eV to 290.25 eV) in Ih-C20@C60. From the differences shown in the XPS spectra of the isomers, we can conclude that there are great differences of the electronic structures and properties among them. Only one peak appears at 289.97 in the free C60 spectrum since all the atoms in this molecule are equivalent. However, the spectrum of free C20 exhibits two wide weak peaks at about 290.36 eV and

5

290.87 eV respectively with an obvious spectral line broaden on the right side of the first feature, which means that the free C20 has a lower symmetry than Ih symmetry. In fact, the symmetry of C20 reduces from Ih to C2h with six non-equivalent carbon atoms because of the possibility of Jahn-Teller distortion [36]. Therefore, the spectrum by the building block approach shows three features a (corresponding the feature in C60 spectrum), b and c (corresponding the features in C20 spectrum) just as mentioned above. By comparing the XPS spectra of C20 + C60 and the three isomers, we can see the significant differences among them which reflect the changes of geometrical and electronic structures of C20 and C60 fullerenes after encapsulation, so the building block approach cannot be well adopted to calculate C 1s XPS spectra for the studied isomers. In addition, the Fig. 3 presents the total XPS spectra of two C20@C60 isomers and Ih-C80 as well as their corresponding decomposed spectra by the carbon atoms in different local environments by the aforementioned classification, so we could shed light on the contributions from different types of carbons to the total spectra. In the spectrum of D5dC20@C60 isomer, the spectral components from top to bottom correspond to the carbon atoms classified by the above from (1) to (6). The feature a at the lowest energy is generated by the carbon atoms belonging to C1 and C6, while the highest peak b in the spectrum originates from the carbon atoms of C3 and C5. Besides, the carbon atoms of C4 give rise to the feature c and the carbon atoms belonging to C2 make contribution to the feather d at the highest energy region. The condition is simpler in the Ih-C20@C60, in which the highest feature b stems from the carbon atoms belonging to the C60 layer and the weaker feature a originates from the carbon atoms of C20 layer. And in Ih-C80, the number of carbon atoms belonging to the corannulene site is 60 and that of carbon atoms belonging to the pyrene site is 20, and the carbon atoms of the corannulene site lead to the strong peak a and the carbon atoms of the pyrene site generate the feather b. Furthermore, the carbon atoms belonging to C2 in D5d-C20@C60 have the highest C 1s IP's compared with the rest types of carbon atoms in C60 part and even the C60 part in Ih-C20@C60; besides, the carbon atoms of C5 in D5d-C20@C60 also shift toward higher energy regions compared to the atoms of C6 and the C20 part in Ih-C20@C60. The strong interactions owing to the short distance between the inner and outer carbon atoms may be responsible for such blue shifts.

Fig. 3. The XPS spectra of two C20@C60 isomers and Ih-C80 and their spectral components for different types of carbon atoms in each isomer. Each individual spectral component is obtained by summing up the spectra of the individual carbon atoms of the same type scaled by their relative abundance. The number of carbon atoms of each type is listed in parentheses.

Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904

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Fig. 4. The calculated C 1s NEXAFS spectra of two C20@C60 isomers and three free fullerenes (Ih-C80, C20 and C60) as well as the building block of C20 and C60.

3.3. NEXAFS spectra The C 1s NEXAFS spectrum describes the excitations of the electron from the core level to unoccupied orbitals, revealing the electronic structure of the molecule to some degree, which makes it be an efficient technique for the isomer identification. The Fig. 4 displays the calculated C 1s NEXAFS spectra of the two C20@C60 isomers, Ih-C80 isomer and the two free fullerenes C20 and C60 as well as their building block (C20 + C60). The NEXAFS spectra of two C20@C60 isomers exhibit apparent differences on both the energy positions of absorption features and spectral profiles. For detailed comparison, we label the major features from a to m in energy-ascending order according to the corresponding energy positions, and the value of energy position for each feature in the spectra of the two C20@C60 isomers, Ih-C80 and C20 + C60 is displayed in Table 4. Four common features of these four systems marked as b, d, i and j appear at around 284.2–284.4 eV, 285.5–285.7 eV, 288.15 eV, and 289.0–289.3 eV, respectively. Whereas, the outlines and relative intensities of above features exist big discrepancies in different clusters. For instance, the feature d in D5d isomer is a small bulge; while in Ih isomer, it is a strong peak. It's noticed that the spectrum of D5d-C20@C60 shows two obvious features c and e (one at about 285.00 eV, another at about 286.25 eV) on either side of the small bulge that are invisible in the spectrum of Ih-C20@C60. Furthermore, at the energy position slightly lower than feature i, D5d isomer appears a single peak (marked as h) with the left-hand line broadening; as for Ih isomer, there are dissociated double-peaks feather (tagged as f and g) with the same intensities. In general, the NEXAFS spectra of the two C20@C60 conformers present such large differences, so the calculated NEXAFS spectra can be a better

method to discern the above two CNOs isomers. One can easily observe that the C20 + C60 shows distinctly different spectrum compared with the two C20@C60 isomers. For example, in the spectrum of C20 + C60, the relative spectral intensities of feature b and feature d are opposite to that in the Ih-C20@C60 and D5d-C20@C60, so the building block approach can also not be used to calculate NEXAFS spectra for this kind of composite system. By comparing the spectra of Ih-C80 and C20 + C60, one can see the huge differences at lower energy region (about 282–285.7 eV for Ih-C80 and 282.6–286.6 eV for C20 + C60). More concretely, the spectrum of Ih-C80 shows a weak feature c at about 283.24 eV and a strongest peak c at about 285.17 eV, however, feature a is absent and the strongest peak b appears at around 284.20 eV in the spectrum of C20 + C60. But interestingly, the two spectra exhibit similar profiles along with the increase of energy (above 286.6 eV for Ih-C80 and above 287.7 eV for C20 + C60). Although the building block approach can't be used to calculate the NEXAFS spectrum of Ih-C80 at the low-energy region, we can depict the Ih-C80 NEXAFS spectrum approximately by building block approach at higher-energy region for the high similarity shown in the spectra of Ih-C80 and C20 + C60 as the increase of energy. In order to analyse the origins of the main features of the total spectra for the two C20@C60 isomers and Ih-C80, we investigated the decompositions of the total spectra (as demonstrated in Fig. 5) generated by different kinds of carbon according to the previous classification. In the spectrum of D5d isomer, we can notice that the carbon atoms of C6 generate the first absorption feature at the lowest energy while the carbon atoms of C2 produce that at the highest energy position. And in the Ih isomer, the energy position of first absorption feature of C20 part is also lower than that of the C60 part. Because the significant spectral features are mainly contributed by the carbon atoms in C60 layer for these three carbon clusters, we consider the contribution of the components of spectra of the C60 layer firstly. As shown in the Fig. 5, in the spectrum of D5d-C20@C60, the small features b, d, and i all arise mainly from the carbon atoms of C3 and the highest peak m at the highest energy position is generated by the carbon atoms of C4, while other significant features usually stem from more than one type of carbon atoms of C60 part. In detail, the features c, e, h, and j are partly constructed by the carbon atoms of C1; besides, the carbon atoms of C2 and C3 give rise to the features h and j respectively and C4 give contribution to both the feature c and feature e. In addition, the feature also results from two types of carbon atoms of C60 layer which are the carbon atoms of C2 and C3. Then, we study the effect of the C20 part for the formation of the total spectrum. The carbon atoms of C5 heighten the feature i and it's a vital reason for the increase of the strength of the feature m; moreover, the carbon atoms of C6 also contribute to the feature c and feature h. It is worth noting that the spectra of the carbon atoms belonging to C3 and C4 present a certain similarity on the spectral profiles, which implies that the carbon atoms of C3 and C4 are in a similar chemical environment (this can also be seen from its structure figure). The distinct differences on the profiles and the energy positions of absorption features of other spectra reflect the NEXAFS spectra sensitivity to the different local environment of different carbon atoms. Furthermore, in keeping with the case of XPS spectra, the C 1s NEXAFS spectra generated by the carbon atoms belonging to C2 and C5 both have palpable blue shifts comparing to the other types of carbon atoms in their same shell. And this further confirms that the strong interactions between inner and outer layer in D5d-C20@C60 can lead to the greater changes in the electronic

Table 4 Energy positions of the major spectral features in calculated C 1s NEXAFS spectra of the two C20@C60 isomers and Ih-C80 as well as the building block of C20 and C60. Molecule

a

b

c

d

e

f

g

h

i

j

k

l

m

D5d-C20@C60 Ih-C20@C60 Ih-C80 C20 + C60

– 282.77 283.24 –

284.37 284.20 – 284.20

285.00 – 285.17 –

285.68 285.50 – 285.45

286.25 – 285.88 –

– 286.62 – 286.02

– 287.14 286.85 286.75

287.42 – 287.67 287.75

288.15 288.15 288.24 –

289.00 289.30 – 289.14

289.81 – 289.46 289.86

– 290.09 – –

290.40 – – –

Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904

Y. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx

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Fig. 5. The NEXAFS spectra of two C20@C60 isomers and Ih-C80 and their spectral components for different types of carbon atoms in each isomer. Each individual spectral component is obtained by summing up the spectra of the individual carbon atoms of the same type scaled by their relative abundance. The number of carbon atoms of each type is listed in parentheses.

structures of the studied composite system. In Ih-C20@C60 and Ih-C80, the total spectra are nearly same with the corresponding spectra of carbon atoms belonging to C1 because the atoms of C1 accounts for the bulk of the molecule. But the atoms belongs to C2 also have a certain influence on both profiles and intensity for the total spectra. In Ih-C20@C60 isomer, the effects of C20 part on total spectrum mainly reflect on the lower-energy region, to be specific, the first feature of total spectrum stems from the atoms of C2 and the intensity of feature b (arises at around 284.2 eV) has been strengthened greatly due to the appearance of the strongest feature in the spectrum of the atoms of C2. Similarly in the spectra of Ih-C80, the strongest peak in C2 part spectrum also further intensifies the highest peak c and the feature i of total spectrum, and the feature arising at 287.67 eV of the C2 spectrum make the line broadening in the spectrum of C1 become a shoulder h of the feature i in the total spectrum.

4. Summary In the present study, the C 1s X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of the two simplest potential buckyonion-like fullerene C20@C60 isomers have been investigated at the density functional theory (DFT) level. We mainly explored the relationship between spectroscopies and structures for the two C20@C60 isomers. Both the XPS spectra and the NEXAFS spectra show remarkable isomer dependence, thus the two X-ray spectra could be effective methods to identify the C20@C60 isomers. From study of the decompositions of the total spectra from carbon atoms in different local environment, the origins of the main features and the spectra sensitivity of different carbon atoms have been clarified. The spectra of the carbon atoms related to the strong interaction between inner and outer layer show a significant blue shift compared to the other carbon atoms in the same layer. By means of the comparison of X-ray spectra of C20 + C60 calculated via using the principle of building block, it can be seen that the building block approach cannot be used to calculate the spectra of the double-shell composite systems due to the strong inter-shell interactions. Moreover, a single layer isomer of C20@C60, Ih-C80 can be distinguished from these two C20@C60 isomers by the C 1s theoretical XPS and NEXAFS spectroscopies.

CRediT author statement Yong Zhou: Conceptualization, Data Curation, Writing-Original draft preparation Juan Lin: Data curation Xue-Ying Nie: Investigation Xiu-Neng Song: Writing-Reviewing and Editing Yong Ma: Project administration Chuan-Kui Wang: Funding acquisition Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant NO. 11804196, 11874242). Thanks to the support of the Taishan scholar project of Shandong Province. References [1] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, C60: Buckminsterfullerene, Nature 318 (1985) 162–163. [2] A.S. Rettenbacher, B. Elliott, J.S. Hudson, A. Amirkhanian, L. Echegoyen, Preparation and functionalization of multilayer fullerenes (carbon nano-onions), Chem. Eur. J. 12 (2006) 376–387. [3] A.A. Voityuk, M. Sola, Photoinduced charge separation in the carbon nano-onion C20@C60, J. Phys. Chem. A 120 (2016) 5798–5804. [4] B. she Xu, Prospects and research progress in nano onion-like fullerenes, New Carbon Mater 23 (2008) 289–301. [5] J. Bartelmess, S. Giordani, Carbon nano-onions (multi-layer fullerenes): chemistry and applications, Beilstein J. Nanotechnol. 5 (2014) 1980–1998. [6] M. Zeiger, N. Jackel, V.N. Mochalin, V. Presser, Review: carbon onions for electrochemical energy storage, J. Mater. Chem. A 4 (2016) 3172–3196. [7] C. Portet, G. Yushin, Y. Gogotsi, Electrochemical performance of carbon onions, nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors, Carbon 45 (2007) 2511–2518. [8] D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna, P. Simon, Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon, Nat. Nanotechnol. 5 (2010) 651–654. [9] Y. Wang, F. Yan, S.W. Liu, A.Y.S. Tan, H. Song, X.W. Sun, H.Y. Yang, Onion-like carbon matrix supported co3o4 nanocomposites: a highly reversible anode material for

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Please cite this article as: Y. Zhou, J. Lin, X.-Y. Nie, et al., Theoretical identification of buckyonion fullerene C20@C60 isomers by XPS and NEXAFS spectroscopy, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117904