Structural Characterization and Identification of Graphdiyne and

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Structural Characterization and Identification of Graphdiyne and Graphdiyne-Based Materials Haihong Bao, Lei Wang, Chao Li, and Jun Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05051 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Structural Characterization and Identification of Graphdiyne and Graphdiyne-Based Materials Haihong Bao, Lei Wang, Chao Li,* and Jun Luo* Center for Electron Microscopy, Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China KEYWORDS: graphdiyne, structural characterization, structural identification, imaging, spectroscopy

ABSTRACT: Graphdiyne (GDY) is a two-dimensional (2D) carbon allotrope consisting of sp2and sp-hybridized carbon atoms. It and GDY-based materials have tremendous application potentials in the fields of catalysis, energy, sensor, electronics and optoelectronics because of their excellent chemical and physical properties. Thus, the explorations to synthesize highquality GDY and GDY-based materials and to reveal the relationship between their structures and properties are of significance, in which their structural characterization and identification are a crucial step. In this review, we focus on advanced structural characterization techniques and results on GDY, GDY derivatives, GDY composites and doped GDY, including scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction

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(XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). This review can provide a systemic understanding of the structural characterization and identification of GDY and GDY-based materials and help their development for highperformance applications.

1. Introduction Carbon has three kinds of hybridized states including sp3, sp2 and sp. These states can form various allotropes, such as graphite, carbon nanotube and graphene with sp2-hybridized carbon atoms, diamond with sp3-hybridized ones, and fullerene with sp2- and sp3-hybridized ones.1-3 Graphdiyne (GDY) is a newly found carbon allotrope, which is composed of sp2- and sphybridized carbon atoms to form two-dimensional (2D) network structure by the conjugate connection with the -C≡C-C≡C- linkage.4-9 The GDY structural model was theoretically proposed for the first time in 1997.10 The first to synthesize GDY was accomplished by Li et al. with a cross coupling reaction on the surface of copper in 2010.11 Moreover, GDY has been revealed to have some interesting properties, such as excellent conductivity,11 more suitable bandgap for semiconductors relative to graphene,12 and capability to work as ideal 2D gas filters.13 These works started and greatly motivated wide researches on GDY. Subsequently, it is found that GDY can be designed into different nanostructures, sizes and shapes. To date, scientists have achieved various GDY materials on diverse substrates, including GDY films,11,14-16 bulk powders,17-19 nanowalls,20,21 nanotubes,22


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nanosheets,23-25 nanowires,26 quantum dots,27 one-dimensional (1D) nanochains,28 2D nanoribbons,28 three-dimensional (3D) frameworks28 and strip arrays,29 which make GDY applicable in vast applications. Notably, the 2D structure of GDY with different stable chemical bonds between carbon atoms causes excellent chemical stability, high electron cloud density, uniformly distributed pore structure, high electrical conductivity and tunable electronic properties, which are all desired in the fields of catalysis, energy, sensor, electronics and optoelectronics. 11,14,16-22,24-33 For instance, GDY films, powders and nanosheets have been employed to fabricate Li-ion battery anodes with excellent rate performance and stability.14,16-18,25 GDY nanowalls featured evenly distributed sharp walls and highly conjugated structure, and they displayed excellent performance of field emission, which is better than those of carbon nanotubes and graphite.21 GDY nanowires showed mobilities up to 7.1×102 cm2 V-1 s-1 and conductivities up to 1.9 × 103 S m-1 at room temperature.26 Furthermore, GDY can be used to produce various composites and derivatives and be doped with dopants, such as GDY-doped perovskites,27,34 GDY-CuO composite nanowires,35 H- and Cl-substituted GDY,36,37 and N-doped GDY.38-40 These materials have exhibited high performances

for

photocatalysis,

electrocatalysis,

optoelectronic

devices

and

Li-ion

batteries.27,35-65 In all the above fields, the structures of GDY and GDY-based materials are the key factors to determine their excellent properties, and thus the structural characterization and identification are significant for their further researches and applications. Herein, this review systemically gives a summary and discussion of the GDY characterization methods and results.

2. Characterization methods of GDY


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The reported characterization methods of GDY can be divided into two groups, imaging and spectroscopy. The imaging group includes scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM). The spectroscopy group contains Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). SEM and AFM are mainly used to characterize the morphological characteristics of GDY, while TEM, EELS, EDS, Raman, XPS, XRD, FT-IR, UV-vis, NMR and XAS are mainly for the crystal-structure, chemical and bonding features.

2.1 SEM SEM is the most widely used characterization method.66 Its distinctive feature is that it can give images with deep depths of focus and high resolutions up to nanoscale and even sub-nanoscale, enabling the clear imaging of complex and rough surfaces/morphologies.67 In general, for GDY and GDY-based materials, SEM is used in the following situations: the various morphologies and sizes of the GDY family can be directly imaged by conventional SEM;11,14,16-22,24-29,68-70 the thicknesses and cross sections of 2D and quasi-2D GDY materials can be characterized by SEM with cross-sectional sample preparations.11,14,20-22,24,71


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Figure 1. (a) Top and (b) side SEM views of GDY nanowalls on a Cu substrate. Reprinted with permission from ref 21. Copyright 2015 American Chemical Society.

For instance, Figure 1a,b show the top and side SEM views of GDY nanowalls. The top view demonstrates that vertically erected GDY nanowalls constitute a continuous matrix containing large voids with diameters in sub-micrometers. The side view shows that the height of these vertical GDY nanowalls is about hundreds of nanometers.21

2.2 AFM AFM is another common method to reveal the morphology of samples. Compared with SEM, its advantages are that it can image samples in air or liquids, and that its accuracy to measure the surface roughness and thickness of an ultrathin 2D/quasi-2D material is high.11,21,23,24,27,29,45,54,7274

For instance, Figure 2 clearly demonstrates that AFM gives the thickness of some GDY

nanosheets to be down to 2.25 ± 0.17 nm, equivalent to around 6 GDY layers.74


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Figure 2. AFM image of a GDY nanosheet on a mica substrate. The inset curve is the height profile along the white line, and it gives the GDY nanosheet thickness to be 2.23 nm. Together with five additional height profiles, the average thickness and standard deviation of the GDY nanosheets are given to be 2.25 and 0.17 nm, respectively. Reprinted with permission from ref 74. Copyright 2018 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.

2.3 TEM TEM with its associated electron diffraction, simulation, EDS and EELS is one of the most powerful methods for characterizing the morphologies, crystal structures, compositions, thicknesses and chemical valence states of carbon-based materials.66,75 For GDY and GDY-based materials, the following five TEM-based techniques have been employed.

2.3.1 Low-magnification TEM (LMTEM)


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LMTEM can be used to identify the different morphologies and sizes of GDY and GDY-based materials.11,15,19,21,23,24,26,42,74,76 As shown in Figure 3a,b, the LMTEM images reveal the distinct morphologies of GDY nanowires and nanosheets. These images can also give the diameters and the areal sizes of GDY nanowires and nanosheets, respectively.26,74

Figure 3. (a) LMTEM image of GDY nanowires. Reprinted with permission from ref 26. Copyright 2015 Royal Society of Chemistry. (b) LMTEM image of a GDY nanosheet. Reprinted with permission from ref 74. Copyright 2018 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.

2.3.2 Electron diffraction (ED) ED is usually used to characterize the crystal structures and stacking modes of GDY.11,15,21,23,26,36,74 Figure 4a is an experimental selected-area ED (SAED) pattern of a GDY nanosheet, which gives the values of interplanar spacings.74 But, the experimental result is not enough to determine the crystal structure, such as the lattice parameters and stacking mode of GDY. Thus, three GDY models with the stacking modes of AA, AB and ABC were built by reported crystallographic parameters (a = b = 0.96 nm, c = 1.095 nm, α = β = 90°, and γ = 120°),


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as shown in Figure 4b-d. Then, their SAED patterns were simulated, leading to the results in Figure 4e-g, which can be used to make a comparison with the experimental result in Figure 4a. The comparison shows that the experimental pattern is consistent only with the simulated pattern of the model with the ABC stacking mode, while it is obviously different from those with the AA and the AB stacking modes. Thus, the crystal structure of GDY can be determined to be with the ABC stacking mode by ED and ED simulation.

Figure 4. (a) Experimental SAED pattern of a GDY nanosheet with the zone axis of [001]. (b-d) Top views of three GDY models with the stacking modes of AA, AB and ABC, in which the layers of A, B and C are indicated in yellow, green and purple, respectively. (e-g) Simulated SAED patterns of the AA-, AB- and ABC- stacked models. Reprinted with permission from ref 74. Copyright 2018 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.

2.3.3 High-resolution TEM (HRTEM)


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HRTEM is a more visible and faster method than ED for characterizing the interplanar spacing and the crystallinity degree of GDY.11,15,23,26,35,74 Figure 5a shows an example, which is the HRTEM image of a GDY/CuO composite.35 From it, the regions of CuO and GDY and their interface are clearly visible, confirming a good combination between GDY and CuO. In addition, this image gives the interlayer spacing of GDY to be about 0.365 nm, which is close to the computationally predicted value, 0.34 nm.77,78 Figure 5b is the HRTEM image of a GDY nanosheet, and its inset is an computationally simulated image made by the theoretical crystal parameters of GDY. It can be seen that the two images are in a very good agreement with each other.73 The above comparisons between experimental and computational results verify the existence and crystal structure of GDY, indicating that theoretical/computational studies are significant in the research field of GDY.74,77,78

Figure 5. (a) HRTEM image of a GDY/CuO composite. Reprinted with permission from ref 35. Copyright 2017 American Chemical Society. (b) HRTEM image of a GDY nanosheet, in which the noise has been reduced by the Wiener filtering method. The inset in (b) is a simulated HRTEM image. Reprinted with permission from ref 74. Copyright 2018 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.


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2.3.4 EDS EDS spectra and mapping can be used to characterize the elemental composition and distribution of GDY and GDY composites.25,35,47,60 It should be noted that EDS mapping is necessarily performed synchronously with scanning TEM (STEM), which is an imaging mode of TEM. For instance, Figure 6 shows a STEM image and its corresponding EDS elemental mapping images of a GDY/CuO composite from the same work in Figure 5a. The EDS mapping images verify the existence of three elements (C, Cu and O) in the composite and demonstrate that the CuO nanowires in the composite were successfully coated by GDY.35

Figure 6. STEM image (a) and EDS mapping images of C (b), Cu (c) and O (d) for a GDY/CuO composite. Reprinted with permission from ref 35. Copyright 2017 American Chemical Society.

2.3.5 EELS


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EELS spectra and mapping are another method for the elemental composition and distribution of samples, similar to EDS. In addition to the similarity, differences exist between EELS and EDS, which are as follows: EELS and EDS are more suitable to light and heavy elements, respectively; EELS can give information about valence/bonding states in samples.79 Thus, EELS is capable of identifying different hybridized states in carbon materials.28,80 For example, in the carbon K-edge of an EELS spectrum of a carbon material, the π-π* and the σ-σ* transitions are ascribed to the sp2- and the sp3-hybridized carbons, respectively.80 But until now, no EELS characterizations on the valence/bonding states of GDY have been reported. EELS can also be used to determine the thickness of a sample. This method is to record an EELS spectrum with the zero-loss peak from a sample and to compare the area integration under the zero-loss peak (I0) to the one under the whole spectrum (It). Then, the sample thickness t is given by the equation t/λ = ln(It/I0), in which λ is the total mean free path of electrons for all inelastic scatterings in the sample.81 For instance, the thickness of the GDY nanosheet in Figure 3b was calculated to be 2.4 nm by EELS.74 The above sections, 2.3.1～2.3.5, are associated with TEM. It should be noted that GDY and GDY-based materials are sensitive to the irradiation of TEM electron beams. To reduce irradiation damages on these materials, researchers often choose low accelerating voltages and low current densities for TEM electron beams. For example, Matsuoka et al. used successfully a TEM electron beam with a low accelerating voltage of 75 kV from a field-emission gun to characterize the stacking mode of GDY nanosheets with a layer number of about 9.23 Li et al. observed the crystal structure of 6-layer GDY nanosheets by using a 120-kV electron beam from a LaB6 gun and a 60-kV beam from a field-emission gun, and their current densities were about


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2.3×104 and 7×103 A m-2, respectively.74 The above conditions are all suitable to GDY and GDY-based materials. In addition, in situ TEM/SEM is useful to find the relationship between the microscopic structure and the properties of a material. Up to now, only one work26 was reported on in situ TEM/SEM for GDY and GDY-based materials. In this work,26 Qian et al. used SEM and electrical probes in SEM to simultaneously measure the structural sizes and the current-voltage curves of GDY nanowires. By these measured results, the average conductivity and mobility of the nanowires were found to be about 1.9×103 S m-1 and 7.1×102 cm2 V-1 s-1, respectively, indicating that the nanowires are excellent semiconductors. More in situ TEM/SEM works are expected for exploring more structure-property relationships of GDY and GDY-based materials.

2.4 Raman spectroscopy Raman spectroscopy is an analytical method with scattering spectra to obtain information on molecular vibrations and rotations.82,83 It can be used to check the layer number and defects of graphene,84,85 the chiralities and diameters of carbon nanotubes,86,87 and the existence and bonding states of GDY.11,14-29, 35,38-40,42,44-52,54,56-61,63-65,87-90

2.4.1 Calculated Raman spectra of GDY In order to reveal the relationship between Raman signals and structural information of GDY, theoretical calculations have been performed in the community of GDY researches. For instance, Zhang et al.91 systematically studied the Raman spectra of GDY by first-principles calculations. Their calculated results about the assignments and relevant features of Raman signals for GDY


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are shown in Table 1 and Figure 7a,b. These results can be used to identify the existence of GDY.

Table 1. Calculated Raman modes and related properties of GDY at the Γ point. Reprinted with permission from ref 91. Copyright 2016 American Chemical Society. freq(cm-1)

irreps

freq(cm-1)

irreps

RRI

0

A2u

529

E2g [R]

1×10-4

0

E1u

556

E1g [R]

1×10-6

94

E2u

684

E2u

119

E1g [R]

769

A2g

132

A2g

774

E2g [R]

161

B1u

788

B1g

172

A2u [I]

894

E1u [I]

176

E1u [I]

956

A1g [R]

282

B1g

1313

B1u

377

E2u

1335

B2u

383

E2g [R]

1×10-4

1364

E2g [R]

434

E1g [R]

1×10-8

1385

E1u [I]

449

E1u [I]

1478

A1g [R]

0.3

466

A2u [I]

1521

E2g [R]

0.03

467

B1u

2142

A1g [R]

1

498

B2u

2168

E1u [I]

505

A2g

2187

B2u

526

B1g

2221

E2g [R]

RRI

1×10-8

1×10-5

0.08

0.006

0.06

a

Phonon frequencies (freq), irreducible representations (irreps) with indicated Raman/infrared activities ([R]/[I]), and relative Raman intensities (RPI) normalized by largest value.


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Figure 7. (a) Calculated Raman spectrum of GDY. (b) Atomic motions of the vibration modes of GDY, where the red arrows indicate the movement directions of the main contributors. Reprinted with permission from ref 91. Copyright 2016 American Chemical Society. (c) Experimental Raman spectrum of GDY. Reprinted with permission from ref 21. Copyright 2015 American Chemical Society.

Specifically, Figure 7a indicates that a calculated Raman spectrum of GDY contains six major peaks: The B peak at 956 cm-1, the G" one at 1364 cm-1, the G' one at 1478 cm-1, and the G one at 1521 cm-1 are mainly attributed to the breathing vibration of alkyne-related and benzene rings, the scissoring vibration of atoms in benzene rings, the vibration of C-C bonds between triply coordinated carbon atoms and their doubly coordinated neighbors, and the stretching of aromatic


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bonds, respectively. The Y peak at 2142 cm-1 originates from the synchronous contracting/stretching of C ≡ C. The Y' peak at 2221 cm-1 is another stretching mode of C ≡ C with the out-of-phase vibrations of different C ≡ C bonds.91

2.4.2 Experimental Raman spectra of GDY An experimentally typical Raman spectrum of GDY samples has mainly four peaks located at around 1383.7 cm-1 (the D band), 1568.7 cm-1 (the G band), 1939.8 cm-1 and 2181.1 cm-1, as shown in Figure 7c. The D band corresponds to the breathing vibration of sp2 carbon domains of aromatic rings. The G band is caused by the first-order scattering of the E2g mode from the inphase stretching vibration of the sp2 carbon lattices in aromatic rings. The peaks at 2181.1 and 1939.8 cm-1 come from the vibration of diyne links (-C≡C-C≡C-)21 and some vibrations related to C≡C,11,21,29,92 respectively. These experimental Raman peaks are broad, and they are not so sharp as those calculated in Figure 7a. This difference should be due to that the crystallized areas in the measured samples are finitely large,21 while those in the calculated models are infinitely large.91

2.4.3 Experimental Raman spectra of GDY derivatives and doped GDY Sulfide GDY (SGDY) cathodes have been reported to be used for Mg-S and Li-S batteries,41 and they showed excellent electrochemical performances, such as high capacity, superior rate capability, stable capacity retention and large coulombic efficiency. As shown in Figure 8a, the Raman spectra of GDY and SGDY both have the peaks of the D and the G bands. Figure 8b indicates that two new peaks appear in the Raman spectrum of SGDY, corresponding to C-S and S-S. It is also found that the Raman spectrum of GDY contains a peak at 2173.7 cm-1, which is corresponding to the diyne links, and this peak disappeared in the Raman spectrum of SGDY.


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These results indicate that the C ≡ C bonds of SGDY had reacted with the S atoms, generating the C-S and S-S bonds. In addition, the intensity ratio between the D and the G bands (ID/IG) in SGDY is larger than that in GDY, suggesting that more defects existed in SGDY due to the introduction of the S dopants. These Raman spectra demonstrate that S and GDY were well integrated in SGDY.

Figure 8. (a) Raman spectra of GDY and SGDY and (b) an enlarged view of the boxed region in (b). Reprinted with permission from ref 41. Copyright 2017 Wiley. (c, d) Raman spectra of (c) pure and (d) N-doped GDY samples. Reprinted with permission from ref 38. Copyright 2017 Springer Nature.


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Similarly, Figure 8c,d show the Raman spectra of pure and N-doped GDY samples.38 The ID/IG value of N-doped GDY is larger than that of pure GDY. Only the spectrum of pure GDY contains a low peak at 2183.2 cm-1, which corresponds to diyne links. These results also indicate that N atoms had been effectively doped into GDY.

2.4.4 Raman mapping on GDY Using Raman signals to perform mapping on a GDY sample, the spatial distribution of GDY can be imaged.29 The SEM images, the EDS line scan result and the Raman spectrum in Figure 9a-c demonstrate that many GDY strips have been synthesized. Further, Figure 9d,e show Raman mapping images of the GDY strips by the G band signals from the Raman spectra, which indicate unambiguously that GDY is distributed in the strips between the Cu regions. Thus, the shape control of large-area GDY films has been realized.


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Figure 9. (a) Low-magnification SEM image of GDY strips. (b) High-magnification SEM image of two GDY stripes and the corresponding EDS line scan taken along the yellow dotted line, showing that the distribution of the C element matches the GDY shape. (c) Raman spectrum of the GDY stripes. (d) Top-view and (e) 3D Raman mapping images of three GDY stripes by the G band signals. Reprinted with permission from ref 29. Copyright 2016 Wiley.

2.5 XPS The basic theory behind XPS is the photoelectric effect, in which the interaction of X-ray photons with a sample surface results in the emission of electrons, namely photoelectrons.93-95 Because each element has a distinctive set of binding energies, XPS can be applied to check the existence of elements and measure their relative contents within the escape depths of photoelectrons in near surface regions.93,96,97


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Figure 10. (a) Full-spectrum X-Ray scan and (b) high-resolution C 1s XPS peak of GDY. Reprinted with permission from ref 17. Copyright 2015 Royal Society of Chemistry. (c) Highresolution C 1s XPS peak of GDY nanosheets decorated by Pt nanoparticles (PtNP-GDNS), in which the sp/sp2 area ratio is 1.87. (d) High-resolution C 1s XPS peak of GDY nanosheets (GDNS), in which the sp/sp2 area ratio is 2.01. Reprinted with permission from ref 45. Copyright 2015 Wiley.

Figure 10a gives a full XPS spectrum of GDY,17 which contains two main peaks at 284.8 and 532.1 eV. They are corresponding to the 1s binding energies of C and O, respectively. After the background was subtracted and the deconvolution was performed, the high-resolution peak of C 1s was obtained, as shown in Figure 10b. It is composed of four sub-peaks, sp2 C-C of benzene rings at 284.5 eV, sp C≡C at 285.2 eV, C-O at 286.9 eV and C=O at 288.5 eV. The area ratio of the sp and the sp2 subpeaks is about 2, confirming that the benzene rings in this GDY sample link with others by -C≡C-C≡C-, which is consistent with the model structure of GDY. Due to the big pore size of the network in GDY, as shown in Figure 4d, the presence of O 1s peak is ascribed to the adsorption of air in the pores. XPS can also be used to characterize the combining degree of GDY composites.35,45,60,61 For instance, GDY nanosheets (GDNSs) decorated by Pt nanoparticles (PtNP-GDNSs) have been fabricated through a facile method of ion-beam sputtering.45 In Figure 10c,d, high-resolution C 1s XPS peaks of PtNP-GDNSs and GDNSs display the same four types of sub-peaks. But, the sp/sp2 area ratio is decreased from 2.01 for GDNSs to 1.87 for PtNP-GDNSs, meaning that a part of C≡C bonds were broken by the interaction between GDNS and Pt.


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XPS has another important function, characterizing the doping degree and doping positions in doped GDY.39,40,65,98 As shown in Figure 11a, the presence of N 1s and C 1s peaks demonstrates the successful doping of N in the N-GDY sample. Compared with pure GDY, the high-resolution peak of C 1s in N-GDY reveals one extra sub-peak of C=N bonding (Figure 11b,c). The N 1s peak was resolved into two sub-peaks (Figure 11d), of which the sub-peak at 398.2 eV is ascribed to pyridinic N and imine 1N. The other sub-peak at 399.4 eV is to imine 2N and pyridinic N. As displayed in Figure 11e, the formation of imine 1N and 2N is due to the substitution of carbon atoms between every two adjacent acetylene bonds by one or two N atoms, and pyridinic N was formed by substituting a carbon atom on a plan defect site or edge with an N atom. Thus, the substituted positions of N atoms can be confirmed through the above XPS spectra analysis.65

Figure 11. (a) Full-spectrum X-Ray scans of GDY and nitrogen-doped GDY (N-GDY). (b) High-resolution C 1s peak of GDY from (a). (c) High-resolution C 1s and (d) N 1s peaks of N-


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GDY from (a). (e) Schematic image of possible N substitution positions for N-GDY, in which the gray and the other spheres represent the C and the N atoms, respectively. Reprinted with permission from ref 65. Copyright 2014 Royal Society of Chemistry.

Table 2. Peak area percentages of different bonds and the sp:sp2 ratios of as-prepared GDY powders (GDY-as) and GDY samples annealed at different temperatures. For example, GDY300 denotes a GDY powder sample through an annealing treatment at 300 °C. Reprinted with permission from ref 15. Copyright 2017 AIP Publishing.

Samples

C=C(sp2)

C-C(sp)

C-N

C-O

C=O

sp:sp2

GDY-as

38.16

37.63

10.93

9.40

3.86

0.99:1

GDY-300

51.57

24.74

11.07

8.12

4.47

0.47:1

GDY-400

37.10

39.55

9.52

9.98

3.83

1.07:1

GDY-600

31.12

46.34

4.83

13.77

3.92

1.49:1

The chemical environment variation of GDY powders after annealing can also be studied by XPS.15 Table 2 lists the peak area percentages of C=C (sp2), C-C (sp), C-N, C-O and C=O bonds and the sp:sp2 ratios of as-prepared GDY powders (GDY-as) and annealed GDY samples, all of which were calculated from XPS. The presence of the C-N bond probably originates from the cross-coupling reaction that took place under ambient N2 when pyridine was present. With the increase of the annealing temperature, the C=C, C-N and C=O proportions decrease, while the CO and C-C proportions and the sp:sp2 ratio increase. The reason why the sp:sp2 ratio increases may be as follows:15 The precursor residues in GDY-as might contain oligomer and polymer. The thermal stability of the oligomer residues is weaker than that of the polymer ones. Thus,


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when the temperature is increased, the relatively unstable oligomer residues are easily oxidized and decomposed and the polymer residues can persist.

2.6 XRD XRD is a traditional but important technique. Through the measurement of diffracted rays in XRD, important structural information of samples can be determined.99

2.6.1 XRD of GDY and GDY composites XRD has been employed to determine the crystal structure, interlayer distance and lattice spacing of GDY.11,24,26,28 The XRD pattern in Figure 12a shows several sharp diffraction peaks, indicating the good crystallinity of the GDY sample.11 In Figure 12b, the broad peak at around 26.5o corresponds to the 0.34 nm interlayer distance, which is the same as the computational value and thus is well supported by the theoretical calculations.77,78 The broadness of the peak is due to the relatively small crystal regions of the sample.24 Figure 12c displays two diffraction peaks at 21.18o and 44.42o for GDY nanowires.26


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Figure 12. (a) XRD pattern of a GDY film. Reprinted with permission from ref 11. Copyright 2010 Royal Society of Chemistry. (b) XRD pattern of GDY nanosheets. Reprinted with permission from ref 24. Copyright 2017 American Chemical Society. (c) XRD pattern of GDY nanowires. Reprinted with permission from ref 26. Copyright 2012 Royal Society of Chemistry. (d) XRD patterns of (i) GDY and (ii) GDY-supported Co nanoparticles wrapped by N-doped carbon layers (CoNC/GDY), of which the latter shows the feature peaks for graphite and metallic Co. Reprinted with permission from ref 48. Copyright 2016 American Chemical Society. (e) XRD patterns and (f) magnified (110) peaks of pristine and GDY-doped perovskite films, in which the GDY concentrations are labeled in (e). Reprinted with permission from ref 27. Copyright 2018 Wiley.

The structure of GDY composites can be explored with XRD.27,46,48,52,55,61 For instance, a GDY-based electrocatalyst consisting of GDY-supported Co nanoparticles wrapped by N-doped carbon layers (CoNC/GDY) has been reported for hydrogen evolution reaction (HER).48 Figure


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12d shows that in the XRD pattern of CoNC/GDY, the characteristic (002) diffraction peak of graphite and the (111) one of Co appear in the GDY background, illustrating the formation of Co-C-GDY composites. Figure 12e displays the typical XRD patterns of pristine and GDYdoped perovskite films.27 The enlarged (110) diffraction peaks (Figure 12f) reveal that the peak position shifted to higher values of 2θ when the concentration of GDY increased. This finding demonstrates that GDY was successfully doped into the perovskite lattice.

2.6.2 Grazing-incidence XRD (GIXRD) Grazing-incidence X-rays have large interactions with sample materials, leading to the high sensitivity of GIXRD for detecting surface information with the interference signals of substrates inhibited. Thus, GIXRD is very suitable for the structure analysis of thin films.100

Figure 13. (a, b) 2D GIXRD patterns of cross-linkable fullerene [6,6]-phenyl-C61-butyric styryl dendron ester (C-PCBSD) films without (a) and with (b) GDY doping, of which the latter is denoted by C-PCBSD:GDY. (c) 1D out-of-plane GIXRD patterns of the undoped C-PCBSD (also denoted as PCBSD) and the C-PCBSD:GDY (also denoted as PCBSD:GD) films. (d, e) 2D


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GIXRD patterns of perovskite films fabricated on C-PCBSD (d) and C-PCBSD:GDY (e). (f) Plots of radially integrated intensities along the ring at q = 10 nm-1, corresponding to the (110) plane of perovskite films on C-PCBSD (the black dashed line) and C-PCBSD:GDY (the red line). Reprinted with permission from ref 101. Copyright 2018 Elsevier.

Figure 13a shows the GIXRD pattern of C-PCBSD at the incidence angle of 0.3°,101 in which no obvious diffraction rings were detected. In contrast, a diffraction ring was observed in the CPCBSD:GDY pattern at 0.3° (Figure 13b). 1D out-of-plane GIXRD patterns can reveal the changes of GIXRD peaks from the surface to the interior of a material. For example, Figure 13c clearly shows one peak at q = 4.22 nm-1, which is from the surface region of the C-PCBSD:GDY sample, but no peaks exist in the pattern of the C-PCBSD sample. These observations indicate that the film orientation of C-PCBSD was highly improved by doping GDY. Additionally, Figure 13d,e displays that the 2D diffraction rings of the perovskite film on C-PCBSD:GDY are sharper than those of the film on C-PCBSD, indicating that the film on C-PCBSD:GDY has a higher crystallinity and a preferential orientation parallel to the substrate. Plots of the radially integrated intensities along the rings at q = 10 nm-1 are shown in Figure 13f, from which a series of relatively sharper peaks, in addition to the peak at the azimuth angle of 90°, were detected for the C-PCBSD:GDY/perovskite film. This finding indicates that this film has a multiple ordered crystal orientation.101

2.6.3 Small-angle XRD (SAXRD)


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Figure 14. SAXRD patterns of a poly(3-hexylthiophene) film doped with GDY nanoparticles (P3HT/GDY) and a pristine P3HT film, in which P3HT/GDY is also written as P3HT/GD. Reprinted with permission from ref 47. Copyright 2015 Wiley.

XRD performed at low angles is termed SAXRD, which can be used to characterize the ordering in organic materials.102 Xiao et al.47 found that the light transmittance of P3HT/GDY is lower than that of pristine P3HT. The reason to cause this difference was explored by SAXRD. In general, the introduction of additives in the P3HT precursor solutions may have a significant effect on the ordering of the polymer chains in the P3HT films as well as their optical properties.103 But, no obvious differences between the SAXRD patterns of P3HT/GDY and P3HT were found (Figure 14), indicating that the introduction of GDY nanoparticles did not affect the self-organization of P3HT polymer chains and thus that the transmittance difference should be mainly ascribed to GDY nanoparticles themselves.47

2.7 FT-IR spectroscopy


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Figure 15. (a) Experimental FT-IR spectrum of GDY. Reprinted with permission from ref 29. Copyright 2017 Wiley. (b) FT-IR spectra of chlorine-substituted GDY (Cl-GDY) and 1,3,5trichlorine-2,4,6-triethynylbenzene monomer (Compound 3). Reprinted with permission from ref 37. Copyright 2017 Wiley. (c) FT-IR spectra of pristine and nitrogen-doped GDY (namely GD) samples, of which the doped samples were prepared by heating pristine GDY at 400 °C, 500 °C, 550 °C and 600 °C in high-purity NH3 mixed with Ar. They are denoted as N 400-GD, N 500GD, N 550-GD and N 600-GD, respectively. Reprinted with permission from ref 65. Copyright 2014 Royal Society of Chemistry. (d) FT-IR spectra of titania (P25), P25-carbon nanotube composite, P25-graphene composite and P25-GDY composite, which are denoted by 1, 2, 3 and 4, respectively. Reprinted with permission from ref 46. Copyright 2012 Wiley.


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A typical FT-IR spectrum of GDY is shown in Figure 15a. The peaks at 1449 and 1603 cm-1 originate from the skeletal vibrations of aromatic rings. The wide bands at 2122 and 2207 cm-1 correspond to the typical C≡C stretching vibration,29 and their intensities are relatively weak due to the perfect molecular symmetry of GDY.11 FT-IR can also be used to study the synthesis mechanism of GDY derivatives.36,37 For instance, chlorine-substituted GDY (Cl-GDY) was synthesized by the cross coupling reaction of 1,3,5trichlorine-2,4,6-triethynylbenzene monomer (Compound 3) on copper foils. Figure 15b shows that the intensity of Cl-GDY's acetylenic C-H vibration peak at 3000 cm-1 is obviously weaker than that of Compound 3, indicating that the cross-coupling reaction of Compound 3 had taken place. Moreover, the weakening of the stretching vibration peak of C≡C (at 2200 cm-1) and the shift of the skeletal vibrations of aromatic rings toward high wavenumbers (1500-1760 cm-1) suggest the increase of conjugated carbon framework. In addition, the FT-IR spectrum of ClGDY shows another two peaks at 900 cm-1 and 1000-1150 cm-1, which may be ascribed to the stretching and bending vibrations of aromatic C-Cl and the asymmetric stretching mode of C-C. The FT-IR spectra demonstrate the formation of a carbon-rich Cl-GDY structure.37 Nitrogen-doped GDY (N-GDY) can be applied as an electrode with a high electrocatalytic activity toward the oxygen reduction reaction in alkaline fuel cells. Its doping was characterized by FT-IR, as shown in Figure 15c. Compared with the pristine GDY, the FT-IR spectrum of each N-GDY showed a new peak at 1620 cm-1, and this peak corresponded to the C=N stretching of imine, suggesting that N had been effectively doped into GDY through the heat treatment.65 It is reported46 that the nanocomposite of titania (P25) and GDY, namely P25-GDY, is a type of photocatalyst for the degradation of methylene blue, and that its activity is better than those of pure P25, P25-carbon nanotube composite and P25-graphene composite. FT-IR was used to


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study the structural origin of the high activity of P25-GDY, and the results are given in Figure 15d. The FT-IR spectrum of pure P25 shows a band around 690 cm-1, which corresponds to the vibration of the Ti-O-Ti bond. In contrast, the peak of P25-GDY near 690 cm-1 shifts towards higher wavenumbers and becomes much broader, suggesting that this peak is from the combination of the Ti-O-Ti and the Ti-O-C vibrations, of which the latter should be located at 798 cm-1. These findings indicate that the Ti-O-C bonds in P25-GDY played an important role in the degradation of methylene blue.46

2.8 UV-vis spectroscopy UV-vis spectroscopy is mainly used to analyze the aromatic benzene rings and conjugated π systems of GDY.21,24 For instance, Figure 16a gives two UV-vis spectra of GDY nanosheets, in which the intensive absorption peaks around 255 nm correspond to the π-π* transition of the aromatic benzene rings. The broad peaks between 350 and 800 nm are attributed to the enhanced delocalization of electrons by extendedly conjugated π systems.24

Figure 16. (a) UV-vis spectra of GDY nanosheets on a Cu substrate (black line) and exfoliated GDY nanosheets (blue line). Reprinted with permission from ref 24. Copyright 2017 American


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Chemical Society. (b) UV-vis spectra of a GDY sample (red line) and its corresponding monomers (black line). Reprinted with permission from ref 21. Copyright 2015 American Chemical Society.

Figure 16b shows that, compared with the UV-vis spectrum of monomers corresponding to GDY, the UV-vis peaks of GDY show significant red shifts. This is due to the enhanced electron delocalization, which indicates that the extendedly conjugated π system was formed in the product through a cross coupling reaction between monomers.21

2.9 NMR spectroscopy 13

C-NMR is powerful for analyzing the carbon species of GDY.28 As shown in Figure 17a, some

of the 13C-NMR peaks of GDY are weak due to its strong conductivity, but can be distinguished. The weak peaks around 50 and 186 ppm are from the sp carbon atoms and the cumulene chains, respectively, while the obvious one at 127 ppm is attributed to the sp2 carbon.


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Figure 17. (a) 13C NMR spectrum of GDY nanochains. Reprinted with permission from ref 28. Copyright 2017 Royal Society of Chemistry. (b)

13

C NMR spectrum of hydrogen-substituted

GDY (HsGDY). (c) Structure diagram of HsGDY. Reprinted with permission from ref 36. Copyright 2017 Springer Nature.

13

C-NMR is also applicable to analyzing the carbon species of hydrogen-substituted GDY

(HsGDY).36 The

13

C-NMR spectrum in Figure 17b displays that there are mainly four types of

carbon atoms in HsGDY: The peaks at 123.1, 135.7, 75.5 and 81.1 ppm correspond to aromatic C-C, aromatic C-H, C (sp)-C (sp) and C (sp)-C (sp2), respectively (Figure 17c).


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2.10 XAS In XAS, near edge X-ray absorption fine structure (XANES) can be used to achieve the structural information of different carbon species.104,105 As a typical example, carbon K-edge XANES has been used to investigate the changes in the bonding states and the electronic structures of two GDY samples that had been exposed to air for one week and three months, respectively.106 The two samples are denoted as GD-1w and GD-3m.

Figure 18. (a) C K-edge XANES spectra of SWNTs (single-walled carbon nanotubes), a graphene oxide sample, GD-1w and GD-3m. (b) C K-edge XANES spectra of the two GDY samples from (a) after they were normalized to Feature A. Reprinted with permission from ref 106. Copyright 2013 American Chemical Society

In detail, Figure 18a indicates that compared with the XANES spectra of graphene oxide and SWNTs, the ones of GD-1w and GD-3m contain some different features around Position A. In the spectrum of GD-1w, a new feature appears, it is denoted as A", and its position is ~0.3 eV higher than that of Feature A. The two features, A and A", are ascribed to the π* excitations of aromatic C-C bonds in carbon rings and C≡C bonds, respectively. In the spectrum of GD-3m,


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Feature A shifts to a position that is 0.3 eV lower than the original position, and it is re-denoted as A'. This shift indicates the conversion of C≡C bonds of GDY into C=C bonds. Features B and C are assigned to oxygenated functional groups and the σ* excitation of C-C bonds in carbon rings, respectively. Figure 18b shows that compared with GD-1w, Feature B of GD-3m is sharper with a higher intensity, suggesting the more oxidation of GD-3m.

2.11 Comparison of the above techniques and their limitations As is mentioned in the beginning of Section 2, the above techniques can be divided into two groups, imaging and spectroscopy. In the imaging group, SEM, TEM and AFM can all let users see directly samples, but differences exist among their resultant images: SEM and AFM images are 3D views of exterior appearances of samples, while TEM images are 2D mixed projections of exterior appearances and interior structures of samples along electron beam directions. Thus, SEM and AFM can easily let users know 3D surface structures and morphologies of samples, but cannot give any information about interior structures of samples except that samples are cut open. In contrast, TEM can give rich information for interior structures but is unable to show 3D views except that complex calculations are used with TEM images to perform 3D reconstructions. In the spectroscopy group, all of Raman, XPS, XRD, FT-IR, UV-vis, NMR, XAS, EELS and EDS cannot give directly images of samples, although some of them (such as Raman, EDS and EELS) can be used to perform signal mapping and give resultant mapping images, which are not direct images of samples. But, they are quite useful, of which XRD can provide crystallographic information and the others can give chemical information of samples. Specifically, FT-IR and Raman belong to molecular vibrational spectroscopies, and the former can give stronger absorption signals for polar functional groups, while the latter is more sensitive for detecting


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nonpolar functional groups. XPS, XAS and UV-vis belong to electron transition spectroscopies, of which XPS is the most sensitive to sample surfaces. Its elemental identification and relative content analysis are very accurate, but its absolute content analysis is quite unreliable. In XAS, XANES is extremely sensitive to chemical structures through samples, such as valence states, density of states and charge transfer. UV-vis is mainly used to identify large conjugated systems, but in general its characteristic signals are not strong enough. Thus, it is generally used as a supplement of other characterization methods. Of NMR,

13

C-NMR can identify the carbon

species in GDY or GDY-based materials. But for electronically conductive samples, NMR signals are weak. In addition, EDS and EELS can also give chemical information of sample compositions, of which EELS is further capable of revealing valence states, and both of them can reach high spatial resolutions up to atomic levels.

3. Conclusions The reported characterization techniques of GDY and GDY-based materials have been summarized and discussed in detail, including: (1) SEM for investigating the surface morphologies, thicknesses and cross sections of GDY and GDY-based materials; (2) AFM for accurately measuring the thicknesses of ultrathin 2D/quasi-2D GDY samples; (3) TEM and its associated ED, simulation, EDS and EELS, which can be used to characterize the morphologies, thicknesses, crystal structures, stacking modes, existences, and elementary compositions and distributions of GDY and GDY-based materials; (4) Raman and its mapping for showing the spatial distributions, existences and bonding states of the materials; (5) XPS for obtaining the elemental compositions, chemical bonds, and hybridized states as well as their relative contents; (6) XRD for studying the crystal structures, crystallinities and ordering; (7) FT-IR for


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characterizing the functional groups and chemical bonds; (8) UV-vis for analyzing the aromatic benzene rings and conjugated π systems; (9) NMR for analyzing the carbon species; (10) XAS for exploring the changes in the structures and electronic structures. All of the above methods contribute to the structural characterization and identification of GDY and GDY-based materials, and if several of them are jointly applied on the structure of a GDY or GDY-based material, the structure can be unambiguously determined, which is of significance for developing the syntheses, modulations, properties, performances and applications of the GDY family.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (C.L.). *E-mail: [email protected] (J.L.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (11604241), the National Program for Thousand Young Talents of China, the Tianjin Municipal


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Structural Characterization and Identification of Graphdiyne and

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