Liang Fang(房良) Yanping Xie(解燕平) Shujie Sun(孫書杰) and Wei Zi(訾威)

1Collaborative Innovation Center of Henan Province for Energy-Saving Building Materials,Xinyang Normal University,Xinyang 464000,China

2Analysis&Testing Center,Xinyang Normal University,Xinyang 464000,China

Keywords: polyynes,carbon chains,single-walled carbon nanotubes

1. Introduction

Carbyne, long linear carbon chain (LLCC), as a pure sp-hybridized carbon material,[1,2]has been studied for decades.[3–5]However,the synthesis of carbyne with infinitelength LLCCs still faces many challenges such as the instability in the ambient,the crosslink reaction between LLCCs,and the high activity from nearly naked carbon atoms.[6]Therefore, the space-confined nanoreactors,[3,7–12]especially some one-dimensional channels like carbon nanotubes(CNTs),have been used as nanoshells to protect and prepare LLCCs. At the same time, short linear carbon chains (polyyne), such as C2nH2(n=1, 2, 3, ...), composed of alternating single and triple bonds through end-capped bonding with hydrogen,have been considered as the most important precursors for the synthesis of LLCCs.[8,12]

Now many reports have successfully synthesized the confined LLCCs in CNTs.[3,7,8,10–12]However,there are still many challenges in this field. One problem is the lack of a simple and efficient technology to prepare polyynes. Although several strategies have reported the synthesis of C2nH2, such as laser-ablation,[13–15]arc-discharge method,[16–18]and chemical synthesis method,[19–21]they still need complex devices and show low-efficiency in the preparation of polyynes. Additionally,an efficient method to regulate the transport of precursors in one-dimensional channels is a key for the synthesis of confined LLCCs. However, the existing methods are powerless to regulate the molecules in confined space.[8,12]Therefore,a facile,low-cost,and efficient approach to prepare C2nH2and research on the filling process of C2nH2into CNTs are urgently needed.

In this paper,we report a simple,low-cost technology for the preparation of C2nH2on a large scale by a civilian laser marking machine.UV absorption spectroscopy and XPS spectroscopy reveal that polyynes, such as C8H2, C10H2, C12H2,and C14H2have been prepared by a laser marking machine.At the same time,we observe the filling process of single-walled carbon nanotubes (SWCNTs) with C2nH2byin-situRaman spectroscopy,further confirming that polyynes have been prepared and successfully filled into the nanoreactor.

2. Experimental details

2.1. Preparation of C2H2n

The polyynes were prepared with a laser marking machine. The laser marking machine is equipped with a CO2laser. The wavelength is 1064 nm,the power is 40 W,and the diameter of the laser spot is about 10μm. A typical preparation process is to scan a 15 mm×15 mm area on the graphite plate inn-hexane solution with a scanning speed of 100 mm/s for 1 hour. During the process, then-hexane solution should be kept at a low temperature to avoid excessive evaporation.Finally, a highly concentrated polyyne solution was obtained by filtering and concentrating process.

2.2. Preparation of SWCNT film and paper

The SWCNTs used in the experiments were prepared by the edips method with an average diameter of 1.3 nm.[22]After purification,[23]2-mg SWCNTs,and 200-mg sodium dodecyl sulfate(SDS)(99%,Alfa Aesar)were added to 100 ml of deionized water. An ultrasonic homogenizer equipped with a 0.5-inch(1 inch=2.54 cm)flat tip at 400 W worked for about 60 min. The dispersion was centrifuged at 1000 rpm for 5 min to remove undispersed SWCNTs,and 80%of the supernatant was stored at a low temperature (4?C) for the preparation of SWCNTs films.

SWCNT thin films were prepared by the vacuum filtration method.[24]Firstly, the SWCNT dispersion was filtered through a nitrocellulose membrane(GSWP02500,Millipore),and then pasted on a 30×30 quartz sheet and dried. The filter membrane was removed by washing with ethyl acetate. Finally, the SWCNT films were treated in the air at 475?C for 60 min.

2.3. In situ filling SWCNTs with polyynes

During the Raman test, 10 μl of polyyne solution was dropped on the SWCNT film. After 1 min, the Raman spectrum of the SWCNT film was recorded at the same point. This process was repeated for many times,and needed to keep the laser of the Raman spectrometer irradiating during the whole process.

2.4. Characterization

The polyynes and SWCNTs filled with polyynes were evaluated by Raman spectrometers(Renishaw InVia-plus and Horiba LabRAM HR evolution) equipped with 532-nm excitation lasers and UV-vis-NIR spectroscopy (Lambda750,Perkinelmer). The SWCNTs were evaluated by a highresolution transmission electron microscope (HRTEM, Tecnai G2 20 200 kV) and scanning electron microscope (SEM,Hitachi SU8220). The chemical states of SWCNTs filled with polyynes and ablated graphite were analyzed with an xray photoelectron spectrometer (XPS, Thermo Scientific KALPHA).

3. Results and discussion

Figure 1(a) shows the photograph of the preparative device of C2nH2inn-hexane solution with a laser marking machine. The energy of the laser focused on the surface of the graphite plate and then ablated the graphite. At the same time, new substances were produced in the ablation process.Figure 1(b) presents the ultraviolet optical absorption spectrum of the solution ablated by a laser marking machine.In the spectrum, four strong absorption peaks at 227, 252,276, and 297 nm, marked in Fig. 1(b), correspond to C8H2,C10H2, C12H2, and C14H2, respectively, according to previous reports.[14,17,18]No other absorption peak is detected in then-hexane solution. These results suggest that the C2nH2molecules can be prepared by a laser marking machine ablating the graphite plate.

Fig.1. C2nH2 prepared by laser marking machine. (a)Photograph of the preparative device for C2nH2,(b)ultraviolet optical absorption spectrum of C2nH2.

Fig. 2. SEM images of graphite plate before and after ablation. (a) SEM image of raw graphite plate,(b)SEM image of graphite plate after ablation.

After laser ablation, the color of the graphite sheet surface in the middle area becomes darker and the reflection is weakened, compared to the edge of the graphite plate without ablation, as shown in Fig. S1 in the supporting information. As shown in Fig. 2(a), before ablation, the surface of the graphite plate exhibits a flake structure, and the edge of the flake is clean. After laser ablation in Fig. 2(b), the surface of the graphite plate still maintains the initial morphology,but many non-uniform nanoparticles appear and adsorb on the edge and surface of the sheets. Relative to the raw graphite plate,the C 1s peak in x-ray photoelectron spectroscopy(XPS)of the graphite plate ablated by laser marking machine has a slight redshift(about 0.1 eV)in Fig.S2 in the supporting information. This shift towards the low binding energy means the presence of the carbon–carbon triple bond(sp-hybridization).These results indicate that some graphite sheets have been ablated by the laser marking machine,and some carbon materials containing sp-hybridized carbon have formed on the surface of the graphite.

Figure 3(a) shows the SEM image of SWCNT film.The bundles of SWCNTs are uniformly distributed, and the SWCNT bundles show high quality without other impurities,which indicates that the filter membrane has been fully dissolved and removed during the preparation of the SWCNT film. As shown in Fig.3(b),the TEM image further confirms the maintenance of high-quality SWCNTs with good structural integrity during the film preparation process. These results show that the surface structure of SWCNTs is not destroyed in the film preparation process, which is extremely beneficial for the filling step of C2nH2.

Fig.3. SEM image of SWCNT film(a)and TEM image of SWCNTs(b).

To further investigate C2nH2, the C2nH2n-hexane solution was dropped on the SWCNT film at room temperature,as shown in Fig. 4(a). Raman spectrometer was used to record the characteristic vibrations of the carbon–carbon triple bond(sp-hybridization)during the continuous dripping process. In addition to the original SWCNTs, in all Raman spectra from Fig. 4(b), an obvious range at about 1800 cm?1–2200 cm?1,corresponding to the stretching vibration of triple bonds in LCCs,which is assigned as C-mode,[25–27]is observed.Moreover,with the gradual drop of C2nH2solution,the intensity of C-mode at 2068.2 cm?1increases. To confirm the stability of C2nH2in the SWCNTs,after the SWCNT film was dropped by the C2nH2n-hexane solution for 50 times,the SWCNTs were treated at 300?C in the air for 2 hours.We can see the intensity of C-mode for the SWCNTs film filled with C2nH2shows no obvious change. In Fig.S3,the relative intensity of C-mode is 0.08 compared to that of G-peak,demonstrating thein-situRaman signal in the filling process mainly comes from the internal C2nH2.These results indicate that a large amount of C2nH2has been successfully filled into the SWCNTs, and with the increase of the amount of C2nH2in the SWCNTs,the Raman intensity of C-mode gradually becomes stronger. Moreover,the most important is that the center position of the C-mode undergoes a significant redshift during the filling process. In Fig.4(b),the center of the C-mode is located at 2068.2 cm?1in the first filling process. However, as the number of drops increases, the center of C-mode gradually red-shifts, and finally stabilizes at 2056.7 cm?1, indicating the frequency of the stretching vibration of C2nH2within the confined space decreases due to the confinement effect.

Fig.4. In situ filling of SWCNTs with C2nH2. (a)Schematic diagram of filling SWCNTs with C2nH2. (b)Raman spectra of C2nH2 filled into SWCNTs.

The x-ray photoelectron spectroscopy (XPS) measurements were performed to evaluate the surface chemical composition of the initial SWCNT and C2nH2-filled SWCNT film.The C2nH2-filled SWCNT film has been treated at a high temperature to remove the C2nH2on the surface of SWCNT film.In Fig.5,high-resolution C 1s spectra of SWCNT and C2nH2-filled SWCNT film were shown. To compare the chemical composition of SWCNT films before and after being filled with C2nH2, we firstly corrected the two curves according to the binding energy position of the carbon–carbon single bond(284.8 eV). In the C 1s spectrum of C2nH2-filled SWCNT film,four peaks centered at 283.9,284.8,285.7,and 286.6 eV can be deconvoluted,which correspond to the sp2carbon,sp3carbon, C=O bond, and C–O bond, respectively. Relative to SWCNTs, the existence of C–O and C=O peaks for C2nH2-filled SWCNTs indicates that the filling process does not affect the surface functional groups of SWCNTs. The reduction of the sp3carbon content may be due to the heat treatment after filling,resulting in a decrease in surface adsorption of carbon.Compared with that of the original SWCNTs,the peak of sp2carbon in the C2nH2-filled SWCNTs has a redshift by 0.12 eV.The shift towards low binding energy implies the transfer of electrons from polyynes to SWCNTs, further confirming the C2nH2has been successfully filled in the SWCNTs.

Fig.5. High-resolution XPS spectra of C 1s for SWCNTs before and after being filled with C2nH2.

4. Conclusion and perspectives

In summary,we described a simple and low-cost method for the preparation of C2nH2by laser marking machine to ablate graphite plate in liquid. Using this method,the polyynes,such as C8H2,C10H2,C12H2, and C14H2can be quickly prepared on a large scale. Employing the fast-scanning capability of the laser marking machine, the destruction of polyynes resulting from local high temperature can be greatly avoided,which is essential for the preparation of polyynes in large quantities. Moreover, as long as the solution is kept at a low temperature,the kilowatt-level,high-power lasers will be utilized in the fabrication of polyynes in large quantities. Additionally,we use anin-situRaman spectrometer to study the filling process of SWCNTs with these polyynes. All results show that C2nH2can be fast filled into SWCNTs under laser irradiation. This provides a new idea for solving the transport problem of precursors in the preparing process of LLCCs by confinement reactions in one-dimensional channels of SWCNTs. Therefore, our reported preparation strategy of C2nH2and the filling method of SWCNTs with C2nH2will provide a new way to study the LLCCs in a confined space.

Acknowledgements

The authors thank Qiuju Zhou, Zongwen Zhang, and Dongli Xu at the Analysis and Testing Center of Xinyang Normal University (XYNU) for materials characterization.Project supported by the Nanhu Scholars Program for Young Scholars of Xinyang Normal University.