ABSTRACT:
Few-layer graphene nanosheets have successfully synthesized by a modified Hummer’s method followed by chemical reduction of exfoliated graphene oxide (GO) in the presence of hydrazine monohydrate. The products were characterized by Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and by photoluminescence (PL) spectroscopy. The results show that as-synthesized GO has a broad emission peak ranging from 400 to 875 nm. Interestingly, the PL emission of GO has been tuned from near-infrared to blue through chemical reduction and the emission is inversely proportional to corresponding size of sp2 carbon cluster, these properties open up new opportunities in bandgap engineering for future optical and electronic devices.
Keywords: Graphene, graphene oxide, thermal annealing, optical property.
1. Introduction
Graphene, a single layer of sp2-bonded carbon atoms arranged in a two-dimensional hexagonal lattice, has gained considerable attention due to its extraordinary electronic, thermal and mechanical properties [1]. Graphene exhibits unique electronic properties such as remarkably high charge-carrier mobility (∽200,000 cm2V-1s-1), high carrier density (∽1013 cm-1), ballistic transport and quantum Hall effect at room temperature. In addition, the large specific surface area (up to 2630 cm2 g-1) and high optical transmittance (~97.7%) in the visible region, which make graphene a promising candidate for various applications [2]. However, the lack of a fundamental bandgap in graphene has significantly limited in its applications in electronics and optoelectronics [3]. Much effort has been made to open the bandgap and modify the electrical properties of graphene, such as using graphene nanoribbons [4], AB-stacked bilayer graphene [5], chemical doping (i.e., substituting C of graphene with B and N) [6], and/or by applying gate voltage [7] had negligible success, providing the bandgap opening up to 200 meV as the best [8].
Recently, graphene oxide (GO) a derivative of graphene, has attracted enormous interest because of its availability and processability over a large area, solution-based processing, low cost, and tunable electrical as well as optical properties through structural modifications. The GO is generally synthesized by the method of Brodie [9], Staudenmaier [10], Hummers [11], or some minor modifications of these methods, which involve the oxidation of graphite to various levels. During the chemical oxidation process, the sp2-C graphite structure transforms into an sp3-C hybrid mode [12], in which the carbon atoms become bonded to oxygen atoms in the form of carboxyl, hydroxyl or epoxy groups tend to form distorted regions due to the high proportion of sp3 C-O bonds (up to 55%) [13]. GO is therefore a two-dimensional network of sp2- and sp3-bonded carbon atoms, in contrast to an ideal graphene sheet which consists of 100% sp2-hybridized carbon. Due to its abundant oxygen-rich functional groups, GO is hydrophilic and could be well-dispersed in water and other solvents to form stable colloidal suspensions, which allows it to be uniformly deposited onto a wide range of substrates in the form thin films. The presence of oxygen-containing functional groups in the structure of GO modify its optical, electrical, mechanical and electrochemical properties [14]. Owing to these advantageous characteristics, GO has a wide range of applications, such as in supercapacitors, Li-ion batteries and transparent conducting electrodes [15], thin-film transistors (TFTs) [16], photovoltaics [17], photodetectors [18], and nonvolatile memory devices [19].
Up to now, various methods have been developed to produce graphene, including chemical vapour deposition (CVD) and epitaxial growth [20], micromechanical exfoliation of graphite [21], epitaxial growth on electrically insulating surfaces such as SiC [22], physical method [23] and chemical reduction or thermal exfoliation of GO [24]. Among them, the chemical reduction of GO, involving graphite oxidation, exfoliation and reduction, is the most efficient approach for producing graphene-based sheets in large quantity and exceptionally low cost. Here we report the fabrication of graphene nanosheest through a modified Hummor synthesis of GO, followed by the chemical reduction in the presence of hydrazine monohydrate. The crystallinity, structure, morphology, and composition of the products were characterized by XRD, FE-SEM, HR-TEM, XPS, FTIR, and Raman spectroscopy. The room-temperature PL spectra of graphene and GO are also studied.
2. Experiments
2.1. Preparation of GO and chemical reduction of GO
GO was synthesized using the modified Hummers method, the details of which have been reported. In a typical reaction, 1g of graphite (99.9%), 12 mL of H3PO4 (98%), and 46 mL of H2SO4 (98%) were stirred together with a Teflon-coated magnetic stirring in an ice bath. Next, 12 g of KMnO4 was slowly added while the temperature was maintained at 0oC. Once mixed, the solution is transferred to a 35 ± 5oC water bath and stirred for 3 h, forming a thick paste. Next, distilled water (90 mL) was slowly dropped into the resulting paste to dilute the mixture, and then the solution was stirred for 1 h while the temperature was raised to 90 ± 5oC. Finally, 90 mL of distilled water was added, followed by the slow addition of 10 mL H2O2 (30%), turning the color of the solution from dark brown to yellow. During this final step, H2O2 (30%) reduced the residual permanganate and manganese dioxide to colorless soluble manganese sulfate. The GO deposit was collected from the GO suspension by high speed centrifugation at 9000 rpm for 30 min. The obtained GO was then washed with 200 mL of HCl (5%), and repeatedly washed with distilled water until the pH = 7. To obtain uniform GO, a low-speed centrifugation at 4000 rpm was first used to remove thick multilayer sheets until all the visible particles were removed (3-5 min). Then the supernatant was further centrifuged at 9000 rpm for 30 min to remove small GO pieces and water-soluble byproduct. Then, the final precipitates were redispersed in 150 mL of distilled water, resulting in GO sheet suspension. The aqueous GO suspension was subsequently reduced to graphene colloid using N,N-Dimethylformamide (DMF, 99.8%) in the presence of hydrazine monohydrate (98%). At first, 25 mL of the obtained GO suspension (~6 mg GO/mL) was dispersed in 125 mL of DMF, followed by a mild sonication for 1 h (in a sonic bath) to achieve a homogenous aqueous GO solution, and then 2.5 mL of hydrazine monohydrate was added. The mixtures were heated at 90 ± 5oC using a water bath for 24 h; a black solid precipitated from the reaction mixtures. Next, to generate a homogenous colloidal suspension of reduced graphene oxide (RGO) in DMF solvent, the obtained RGO was diluted 6 times with DMF (resulting concentration ~ 0.2 mg/mL), and then the mild sonication was applied for 60 minutes to order to obtain stable and homogenous RGO dispersion. It was observed that the RGO was stable at room temperature for a few weeks. This stable period allowed sufficient time for sample preparation and characterization steps.
2.2. Characterization
The morphology and structures of GO and RGO samples were characterized by using a field-emission scanning electron microscope (FESEM, JSM-6700, JEOL Ltd., Tokyo, Japan) operated at an accelerating voltage of 12 kV. Transmission electron microscope (TEM) images were taken on a JEM-100CX II (JEOL Ltd., Tokyo, Japan) electron microscope operated at 100 kV. The Raman spectra were taken using a Jasco Laser Raman Spectrophotometer NRS-3000 Series, with an excitation laser wavelength of 532 nm, at a power density of 2.9 mW·cm-2. X-ray photoelectron spectroscopy (XPS, VG Multilab ESCA 2000 system, UK) was performed to analyze the elemental compositions and the assignments of the carbon peaks of the samples, and spectra were recorded using a monochromatized AlKα X-ray source (hν = 1486.6 eV). The FTIR spectra (500-4000 cm-1) were obtained using a Nicolet IR100 FTIR spectrometer. Photoluminescence (PL) measurements were conducted at room-temperature using a He-Cd laser line with the excitation source of 325 nm.
3. Results and Discussion
Fig. 1(a&b) show typical FE-SEM images of the as-made GO nanosheets. From Fig. 1a, the thin wrinkled accordion and/or worm-like structure morphology of the GO sheets can be seen. This material consists of randomly aggregated, thin and wrinkled sheets, being loosely associated with each other. The sizes of GO sheets are in the range of 3-7 μm, with a few of them larger than 10 μm. Fig. 1b displays a high-resolution FE-SEM image of the GO. It is clearly seen that the GO sheets mainly consist of double-and tri-layer graphene, and some of them are folded at the edge of nanosheets. Further observation of morphology has been studied by TEM. As shown in Fig. 1(c), the transparent GO nanosheets exist in the form of thin few-layer grephene with folds and wrinkles; these are characteristic of thin and two-dimensional (2D) GO.
Figure 1: FESEM (a & b) and (c) TEM images of GO, respectively
Micro-Raman spectroscopy is widely utilized for analyzing the structural changes of carbonaceous materials, including disorder and defect structures, defect density, and doping levels [25]. Fig. 2(a-c) shows the micro-Raman spectra of the raw graphite, GO and RGO, respectively. The Raman spectrum of the raw graphite displays the prominent G band at 1582 cm-1 which is commonly ascribed to the first-order scattering of the E2g mode observed for sp2-carbon domains at the Brillouin zone center and a very weak D band at 1359 cm-1 is related to sp3-hybridzed, structural defects, grain boundaries, carbon amorphous or edge planes that can break the symmetry and selection rule [26]. In the Raman spectrum of GO, the G band is broadened and shifted to 1597.0 cm-1 whilst the D band at 1352.0 cm-1 becomes prominent, indicating the destruction of sp2 conjugated system and formation of structural disorders in the carbon lattices due to harsh oxidation by strong acids during the synthesizing process. After GO was chemically reduced to RGO, the D became narrower and more prominent while the G band downshifted from 1597.0 to 1594.0 cm-1, might be due to increase of the number of sp2 carbon in the graphene sheets. The intensity ratio of D band to G band (ID/IG) is commonly used to evaluate the degree of defects and average size of the sp2 domains [27]. Different ID/IG values of graphite, GO and RGO samples were shown in Table 1. It is found that the ID/IG ratio of GO is much higher than that of graphite, clearly indicating the presence of a substantial disorder and large defects. After chemical reduction, the ID/IG ratio of RGO continuously increased, suggesting that more structural defects were introduced during the process. In this reduction step, the oxygenated groups are partially reduced to re-establish the conjugated graphite network, and the vacant lattice sites formed by the removal in the form of CO or CO2 during oxidation remain unchanged. Commonly, GO has an ID/IG value of less than 1, and RGO has a value more than 1. This phenomenon is caused by decreased sp2 domains due to reduced size of GO sheets after reduction [28]. On the other hand, the incomplete recovery of sp3 defects after reduction reactions also could affect ID/IG ratio increase. In our study, the ID/IG ratio of GO was 0.90 and that of RGO was 1.12, which is in reasonable agreement with the literature. The ID/IG ratio increase can be due to changes in the degree of reduction, as reported in many previous studies [29]. The size of the sp2 carbon domains (designated as La) can be estimated from the ID/IG ratio using the empirical Tuinstra-Koenig relation [26]:
La = 4.35 (ID/IG)-1
The calculated size of sp2 domain is estimated to be ~43.5, 4.8 and 3.9 nm for graphite, GO, and RGO, respectively, as shown Table 1. After the harsh oxidation process, the size of sp2 carbon domain dramatically decreased from ~43.5 nm for graphite to ~4.8 nm for GO, indicating destruction of the sp2 atomic structure graphite. After chemical reduction of GO sheets, the size of sp2 carbon domain continuously reduced to ~3.9 nm, suggesting that the chemical reduction can easily cause nucleation of sp2 domains in the sp3 matrix and the density of small sp2 nucleus increased, decreasing the average size of sp2 domains.
Figure 2: Rama spectra of (a) raw graphite powder, (b) GO and (c) RGO samples. (d) FTIR spectra of dried GO and RGO samples. High resolution XPS spectra of
(e) GO and (f) RGO samples
Fig. 2d shows the typical FTIR spectra of GO and RGO samples. The FT-IR spectrum of GO exhibits a strong and broad band at 3400 cm-1, which is due to the O-H stretching vibration. The band at 1715 cm-1 is related to the C=O stretching motions of COOH groups at the edges of the sheets. The band at 1627 cm-1 can be due to the O-H bending vibration of absorbed water. The band at 1580 cm-1 contained a peak corresponding to aromatic C=C bonds and is assigned to skeletal vibrations from un-oxidized graphitic domains. The peaks at 1397 and 1071cm-1 may be attributed to the variations of tertiary C-OH groups and C-O, respectively. In the FT-IR spectrum of the RGO nanosheets, the peaks at 1715, 1397 and 1054 cm-1 disappeared, and broad peak at 3410 cm-1 was markedly reduced, indicating the removal of the hydroxyl and carboxylic acid groups. The intensity of peak at 1633 cm-1 (O-H) was significantly decreased but still detectable. It did not completely disappear because of the mild reaction conditions. In the meanwhile, the shift of the peak for aromatic C=C from 1580 cm-1 in GO to 1557 cm-1 in RGO appears, indicating more defects were added during chemical reduction may modify the sp2 carbon network.
XPS is a powerful tool to identify the elemental composition in bulk materials. Furthermore, by analysis of binding energy (BE) values, we can detect the presence of any oxygenated groups. As shown in Fig. 2e, a typical curve fitting of the C1s XPS spectrum of GO yields components at 284.8, 286.0, 287.3, 288.9 and 289.9eV, corresponding to binding energy of sp2 carbon bonds (C=C), sp3 carbon with C-OH (hydroxyls), C-O-C (epoxy/ether), C=O (carbonyl/ketone), O=C-OH (acid/ester) groups, respectively [30]. This result implies that the GO contains large amounts of sp3 carbon bonds, which resulted from harsh oxidation and destruction of the sp2 atomic structure graphite. Chemical reduction of GO results in significant restoration sp2 carbon networks but is still unable to completely remove all the oxygen functional groups. After chemical reduction process, the C/O atom ratio changes from 2.26 for GO, to 7.02 for RGO, indicating the efficient de-oxygenation of GO and the formation of graphene. Fig. 2f shows curve fitting of the C1s XPS spectrum of RGO, it is observed that the peak at 289.9 eV disappeared, while four remaining peaks shifted into 284.6, 285.8, 286.7 and 288.6eV. It is clearly seen that the contribution of C=C band abruptly increases from 40.5 % for GO, to 72.5% for RGO. An obvious narrowing of C=C band is also observed, evidencing a more ordered structure for RGO. The increase of C=C contribution for RGO is mainly due to the decreased amount of O=C-OH, C=O and C-O-C, indicating that the mild reduction of GO was efficient at removing the oxygen-containing functional groups, especially epoxy and hydroxyl groups. Conversely, the contribution of the peak at 285.8 eV slightly increased, resulting from C-N bond formation during hydrazine reduction.
The optical characteristics of synthesized GO and RGO samples have been studied through PL spectroscopy. In Fig. 3a, the typical PL spectrum of GO at room temperature exhibits a broad PL response from 400 nm to 875 nm [31], which is attributed to the recombination of electron-hole pairs localized within small sp2 carbon clusters embedded within a sp3 matrix and to the presence of oxygen-containing functional groups. This band can be deconvoluted into four Gaussian-like peaks at 542, 654, 747 and 832 nm, in the green, yellow-orange, red and near infrared (NIR) regions, respectively. The luminescence from GO originates from defect states related to vacancies, interstitial atoms, and various functional groups on the graphene surface and edge [32]. The structure of GO consists of sp2 carbon (C) clusters of a few nm dispersed in a defective carbon lattice [33]. Our XPS result in Fig. 5a indicates a large fraction of C atoms present in hydroxyl, carbonyl, acid/ester and epoxy bondings. In GO, containing a mixture of sp2 and sp3 bonded C atoms, the emission is mainly controlled by π–π* states, which lie within the σ-σ* gap [34]. The oxygen containing functional groups are attached to a large fraction of distorted C atoms in GO. Therefore, the origin of PL peaks at different energies are attributed to the transition in disorder-induced localized states [35] or bond alteration within GO plane giving rise to inter valley scattering [31]. The appearance of broad PL peak from as-prepared GO is in agreement with the results reported in the literature [31] and that predicted by the DFT calculations [36]. The board PL suggests a dispersion of hard gaps, which may arise from bond alternation within the GO plane giving rise to intervalley scattering [31]. This can be due to the fact that the bonding of many O atoms and oxygen-containing functional groups to graphene in GO lead to the transfer of resonance energy from the O to the C sp2 clusters in the graphene lattice, thus the radiative recombination rate is increased, yielding broad emission features in the PL [37]. In contrast, the PL emission can be affected by induced changes in the electronic structures and local density of states (DOSs) within the π and π* gap, indicating that the O atoms play an important role in the PL features [38]. The variation of the PL emission of GO can result from the incorporation of oxygen (O) into C sites, forming C-O bonds and oxygen-containing functional groups in the graphene lattice, where these sites may act as energy traps. In Fig. 3b, the PL spectrum of RGO shows a band ranging from 400 to 750 nm which has become narrower than that seen in GO and lower PL intensity compared to that of GO. The PL spectrum of RGO on deconvolution yields three Gaussian-like peaks at 490, 590 and 680 nm. It is found that as the size of sp2 C cluster was decreased, the shift in PL emission peak towards to blue wavelength has been observed. A possible explanation would be the RGO has a smaller sp2 C cluster than GO, resulting in a larger bandgap due to a strong carrier confinement effect. These results demonstrate that the PL emission is strongly depending on the sp2 C cluster sizes which are well agreement with previous report [39].
Figure 3: Room temperature PL spectra of (a) GO and (b) RGO samples
4. Conclusion
Few-layer graphene nanosheets had been synthesized by a modified Hummer’s method followed by chemical reduction of exfoliated GO in the presence of hydrazine monohydrate. As-prepared GO and RGO samples were studied by using XRD, Raman spectroscopy, FESEM, TEM, XPS, FTIR, and PL spectroscopy. The results indicate that as-synthesized GO has a broad emission peak ranging from 400 to 875 nm. Interestingly, the PL emission of GO has become has become narrower and shifted from NIR to blue after chemical reduction, and the emission is inversely proportional to corresponding size of sp2 carbon cluster, these properties open up new opportunities in bandgap engineering for future optical and electronic devices.
ACKNOWLEDGEMENT:
This work was supported by Ho Chi Minh City University of Technology - Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number T-CNVL-2018-13, and by VNU-HCM under grant number B2020-20-07.
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Cấu trúc và tính chất quang của Graphene tổng hợp
theo phương pháp khử hóa học từ Graphene Oxide
PGS.TS. TRẦN VĂN KHẢI (1,2)
(1) Khoa Công nghệ vật liệu, Trường ĐH Bách khoa Thành phố Hồ Chí Minh,
(2) Đại học Quốc Gia Thành phố Hồ Chí Minh
Tóm tắt:
Những tấm nano graphene được tổng hợp thành công từ những tấm GO được chế tạo theo phương pháp Hummor cải tiến kết hợp khử hóa học trong sự hiện diện của hydrazine monohydrate. Mẫu tổng hợp được đặc trưng bởi nhiều phương pháp phân tích hiện đại, như: quang phổ Raman, kính hiển vi điện tử quét phát xạ trường (FESEM), kính hiển vi điện tử truyền qua (TEM), quang phổ quang điện tử tia X (XPS), quang phổ hồng ngoại biến đổi Fourier (FTIR), và bằng quang phổ phát quang (PL). Kết quả đo cho thấy GO được tổng hợp có cực đại phát xạ rộng từ 400 đến 875 nm. Điều thú vị là, phát xạ PL của GO có thể được điều chỉnh từ vùng cận hồng ngoại chuyển sang vùng màu xanh thông qua quá trình khử hóa học và phát xạ tỷ lệ nghịch với kích thước của cụm carbon sp2 tương ứng, những đặc tính này mở ra cơ hội mới trong kỹ thuật điều khiển năng lượng vùng cấm của GO, hướng tới các ứng dụng tiềm năng cho các thiết bị quang học và điện tử trong tương lai.
Từ khóa: Graphene, graphene oxide, khử hóa học, tính chất quang.
[Tạp chí Công Thương - Các kết quả nghiên cứu khoa học và ứng dụng công nghệ, Số 11, tháng 5 năm 2020]
