石墨炔制备与发光性能

石墨炔制备与发光性能
ZHENG Yong-ping;FENG Qian;TANG Nu-jiang;DU You-wei
【摘要】采用交叉偶联反应制备出石墨炔.微结构结果表明,所制样品含有诸多富氧官能团,O/C原子比约为0.1741,厚度约为1μm.研究了石墨炔样品的光致发光特性,发现石墨炔在397 nm处显示出强的紫外(UV)光致发光,并且在280 nm处具有光吸收.其发光是因石墨炔中富氧官能团诱导电荷转移所致.石墨炔作为新型的人工合成碳同素异形体,将在光致发光方面具有潜在的应用.
【期刊名称】《新型炭材料》
【年(卷),期】2018(033)006
【总页数】6页(P516-521)
【关键词】石墨炔;光致发光;交叉偶联反应
【作者】ZHENG Yong-ping;FENG Qian;TANG Nu-jiang;DU You-wei
【作者单位】;;;
【正文语种】中文
【中图分类】TQ127.1+1
1 Introduction
Graphene and related materials are considered as promising photoluminescence (PL) materials, which can beused in fluorescent bioimaging stem owing to their biocompatibility[1-2]. The ideal graphene
whichconsists of 100% sp2-hybridized carbon atoms and is a zero-band-gap material, is highly unlikely to emit luminescence. For graphene oxide (GO), the bonding structures are much more complicate than that in graphene. Most in-plane carbon atoms in GO sheets preserve sp2-hybridized structures, and can covalently bond with oxygen in the forms of epoxy and hydroxyl groups. Moreover, the edge carbon atoms can be bonded with oxygen in the forms of carboxyl and carbonyl groups. This kind of the hybrid sp2/sp3 structure opens up the possibility for PL in graphene-based materials by recombination of electron-hole (e-h) pairs localized within small sp3 carbon clusters embedded within a sp2 matrix. Thus, by controlling the fraction of sp2/sp3 bonds, the PL can be tuned from the visible to near-infrared wavelength range[3-4].
Graphdiyne (GDY), another 2-dimensional periodic carbon allotrope, is greatly different from the case of the hybridized orbital in graphene which only has single or double bonds. GDY can be built from triple- and double-bonded units of two carbon atoms[5-9]. Recently, this kind of new sp-sp2 hybrid structure has aroused great interests owing to its potential new physical and chemical properties. In 2010, GDY was first synthesized by Li et al[10-12]. Thereafter, GDY motivated enormous investigations in various areas such as the electronic properties[13-14], Li-battery storage[15-16], metal catalytic[17], solar cell[18]and diode device[19]. Theoretically, GDY is a kind of two self-doped non-equivalent distorted Dirac cone materials, which has been predicted to possess superior electronic properties to graphene[5]. Furthermore, the sp-sp2 hybrid bonds construct the
structures of rings and chains, which leavesmore complicated π bonds in planar, can be further functionalized through substitution reactions with little damage to the extended π -electron conjugation system. Moreover, this kind of flexible structure can be utilized in composite materials for improving performance. For example, GDY/ZnO composites have been demonstrated to have excellent performance as ultraviolet (UV) photo detectors[20]. However, the PL properties of GDY are still lacking. Therefore, the investigation of the optical properties of GDY is urgent and of great significance.
Herein, we report the PL properties of GDY prepared by cross-coupling reaction. The results show that the as-prepared GDY can emit strong UV PL at 397 nm when excited by UV radiation. Our findings can extend the knowledge of GDY as a new kind of carbon-based PL material.
2 Experimental
GDY was synthesized on the surface of copper foilvia a cross-coupling reaction using hexaethynylbenzene as the precursor[10]. The monomer of hexaethynylbenzene was synthesized by adding tetra- butylammonium fluoride into a hexakis[(trimethylsilyl)- ethynyl]benzene solution in tetrahydrofuran for 10 min in an ice bath. The GDY was grown on the surface of copper foil in the presence of pyridine by a cross-coupling reaction of the monomer of hexaethynylbenzene for 72 h at 80 ℃ under nitrogen atmosphere. In the reaction, the copper foil was used not only as a catalyst for the cross-coupling reaction, but also as a substrate for growing the GDY. After reaction, a black film appeared on the copper foil,
and some powder was left after pyridine was evaporated. For purification, both the copper foil and powder was washed sequentially with acetone, hot dimethylformamide and ethanol. Then, the film was peeled off by ultrasonication in deionized water (DI) for 2 h. To remove metallic ion and other inorganic salt, the powder was washed sequentially by 2 mol/L HCl, 4 mol/L NaOH, DI water and ethanol for several times.
The morphology of the sample was investigated by transmission electron microscopy (TEM, JEM-2100, Japan) and scanning electron microscopy (SEM, S-3400N, Japan). Raman spectroscopy (HORIBA, UK) can be used as a finger printing and quality measurement to characterize GDY structures. Raman spectrum of the sample was measured on glass base under 532 nm at room temperature. To avoid damaging the sample, the incident power was reduced to 0.5 W. Chemical structure of the sample was characterized by X-ray photoemission spectroscopy (XPS) on a PHI-5000 Versa Probe using Al Ka radiation. The UV-vis absorption and PL spectra were measured at room temperature on a Shimadzu UV-2450 spectrophotometer and a HORIBA FluoroLog-3 fluorescence spectrophotometer, respectively.
3 Results and discussion
3.1 Morphology and microstructure of GDY
Fig. 1a shows the SEM image of GDY film grown on a copper substrate. One can find that GDY exhibits a clear uneven sheet structure with an irregular edge, and the thickness of the GDY sheet is about 1 μm. T he TEM and HRTEM images (shown in Fig. 1b and the inset) show that after peeled
off from the copper foil, the GDY sheets are multilayer. Obviously, the ultrasonic process is difficult to exfoliate multilayer GDY into single- or few-layer sheet. The selected area electron diffraction (SAED) pattern was taken from the dash circle area (shown in Fig. 1c). As found, only ring pattern was observed in the sample, which corresponds to the crystal plane with a d-spacing of 0.84 nm. The result agrees well with the theoretical d-spacing of 0.821 nm with a lattice constant of 0.946 nm[21]. The theoretical crystal structure is shown in Fig. 1d.
Fig. 1 (a) SEM image of GDY on copper foil surface, (b) Typical TEM image (Insert is the HRTEM image), (c) SAED pattern taken from dash circle area in (b), the ring corresponds to crystal planes and (d) GDY crystal structure with the lattice of 0.944 nm, the dash line indicates a unit cell.
The Raman spectrum of GDY is shown in Fig. 2a. As shown, two prominent peaks can be observed, one is the D band at 1 370 cm-1 and the other is the G band at 1 578 cm-1, corresponding to the first-order scattering of the E2g mode of in-phase stretching vibration and the breathing vibration of sp2-hybridized carbon domain in aromatic rings, respectively, similar to the result reported[10]. The ratio of the intensity of D and G bands of GDY is 0.84, indicating that there are high-content defects in the
sample.Moreover, there are two extra weak peaks at 2010 and 2 140 cm-1, which can be attributed to the vibration of conjugated diyne links (—CC—CC—). Namely, GDY wassuccessfully prepared via the cross-coupling reaction.
XPS was used to characterize further to explore the chemical environment
of the GDY. As shown in Fig. 1c, the XPS full range spectrum shows that there are the C 1s, N 1s and O 1s peaks, which appeared at 283.9, 397.0 and 530.9 eV, respectively. It is found that the C, O and N atomic percentages are 82.12%, 14.30% and 3.58%, respectively. It is clear that thehigh O percentage indicates that there are O-rich groups such as -OH, -CHO and epoxy groups in as-prepared GDY. To identify the functional groups, the XPS peaks were deconvoluted into different components and quantitatively interpreted after the Shirley background was subtracted. Notably, each peak is strictly fixed at the same full width half maximum and fitted to Voigt functions, followed by a mixture of 25% Lorentzian and 75% Gaussian component. The C 1s peak was deconvoluted into five sub-peaks at 284.5, 285.2, 286.0, 286.6 and 288.8 eV, which can be assigned to C 1s orbital in C—C (sp2), C—C (sp), C—N, C—O and bond (shown in Fig. 2c)[10,15,22], respectively. The peak integration of each component is shown in Fig. 2d, which clearly shows the distribution of different functional groups. It is known that (i) the sp:sp2 ratio is used as the fingerprint for a structure consisting of benzene rings linking together by diyne moieties, and (ii)the sp:sp2 ratio approaching to 2 means the perfect structure of a GDY sheet. However, it is found that ratio of as-prepared GDY is only 0.88, much lower than 2. It indicates that there are many impurities or vacancies exist in as-prepared GDY (shown in Fig. 2d). Actually, as mentioned above, there are different kinds of functional groups in as-prepared GDY, such as epoxy and hydroxyl.
Fig. 2 (a) Raman spectrum, (b) XPS spectrum and (c) High-resolution C 1s
spectrum. The circle symbols, line and other symbol lines are the measured, fitting curve and fitted single peaks, respectively and (d) Chemical bond percentages calculated from the high-resolution XPS C 1s spectrum.
3.2 PL properties of GDY
Fig. 3a is the UV-vis spectrum of GDY. It is found that there is an absorption peak at ca. 280 nm (5.14 eV), in consistent with the theoretical prediction of 4.93 eV using the GW+RPA method by considering quasiparticle effects[23]. This peak can be resulted from the transition around the Van Hove singularities at the M and K point. To explore the PL properties of the GDY, a detailed PL study was carried out using different excitation wavelengths ranging from 310 to 370 nm. As shown in Fig. 3b, there is a strong PL emission peak at ca. 397 nm as the excitation wavelength ranged from 310 to 330 nm. However, as the excitation wavelength increases further, the peak is weakened and broadened dramatically along with the red-shift.
Fig. 3 (a) UV-vis absorption spectrum and (b) PL spectra at different excitation wavelengths. Insert is PLE spectrum with the detection wavelength of 397 nm.
To investigate further the PL emission behaviour, we recorded the PL excitation (PLE) spectrum under 397 nm excitation. As shown in the insert in Fig. 3b, the PLE shows one strong peak at 320 nm. One can considerthat the strong peak at 320 nm can be regarded as the transitions between σ and π orbitals[24]. As known, GDY theoreticallyhas a direct band gap of ca.
0.55 eV. Thus, the band gap transition is tiny and corresponds to near-
infrared emission. Thus, it can’t be the source of PL emission observed. Note that some studies have demonstrated that PL emission in graphene quantum dots can attribute to the size because quantum confinement effect can open the band gap. Especially, the effect can be greatly enhanced when the size of quantum dots is reduced to 10
nm[25].However, TEM analysis shows that our GDY sample is thick and has a large size, suggesting that the size effect should be excluded in our GDY sample.It is well known that the possible origin for PL emission in GO is the oxygen-rich functional groups. As reported, the oxygen-rich functional groups on graphyne sheet can lead to significant charge transfer from C to O and, thus can open a band gap in graphyne[26]. Actually, the XPS results clearly show that there are many oxygen-containing functional groups in our GDY sample.Reasonably, one can proposed that the charge transfer from C to oxygen-containing functional groups in GDY sheets is responsible for the UV PL, similar to the PL mechanism in N-doped graphene quantum dots[24,27].
4 Conclusions
In summary, GDY was synthesized by a cross-coupling reaction. The Raman spectrum and morphology show that as-prepared GDY has a 2D crystalline structure. The results show that as-prepared GDY has UV PL at ca. 397 nm. It is proposed that the oxygen-rich functional groups in GDY sheet can lead to charge transfer and thus open the band gap, and which can be the origin of PL emission. The results show that GDY can be a new kind of carbon-based PL material, and can have many potential
applications such as UV photodetectors and bioimaging materials. References
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