Horizontally Aggregation of Monolayer Reduced Graphene Oxide Under Deep UV Irradiation in Solution
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Graphene has been widely used in novel optoelectronic devices in decades. Nowadays, fabrication of large size monolayer graphene with spectral selectivity is highly demanded. Here, we report a simple method for synthesizing large size monolayer graphene with chemical functionalized groups in solution. The few layer nano-graphene can be exfoliated into monolayer nano-graphene under short time UV irradiation in protic solution. The exfoliated monolayer nano-graphene could experience deoxygenation during long time UV exposure. At the same time, the edge of nano-graphene could be activated under deep UV exposure and small size nano-graphene sheets further aggregate horizontally in solution. The size of aggregated rGO increase from 40 nm to a maximum of 1 μm. This approach could be one promising cheap method for synthesizing large size monolayer reduced graphene oxide in the future.
KeywordsUV exfoliation Aggregation of rGO Monolayer Few layer nano-graphene
Atomic force microscopy
Chemical vapor deposition
Transmission electron microscope
Graphene is a potential material for ultrathin optoelectronic and photodetection devices because of its high carrier mobility and high optical transparency [1, 2]. The key to the high photoresponse of graphene-based devices is the fermi level shifting that induced by the injunction of carriers . With the development of chemical vapor deposition (CVD), growth of large size graphene as well as fabrication of graphene-based devices becomes convenient. However, graphene-based photoresponse device usually has weak absorption and poor spectral selectivity. The common method used to overcome this drawback is hybridizing graphene with quantum dots , plasmonic nanostructure , or other 2D materials with energy gaps  in order to achieve photo-induced carrier injection. Although CVD method promotes the fabrication of growth of large size graphene, the deposition process commonly happens in extreme environment, such as high vacuum, highly selected substrate, and so on. This limits the enlargement fabrication for commercial manufacture. New and low-cost methods are urgent to be developed. Solvent-mediated exfoliation for few layer flakes is one of the efficient and low-cost methods in graphene fabrication [7, 8, 9, 10, 11, 12, 13, 14, 15]. The most widely used method is modified Hummer’s method. The graphite can be oxidized and exfoliated into few layer graphene. Meanwhile, graphene fabricated via chemical oxidized exfoliation usually contains various functional groups which can enhance the optical absorption and spectral selectivity. On the other hand, the oxidized exfoliation process usually damages the crystallinity of sp2 domain , which requires extremely high temperature for recovery. Although the thermal reaction process could recover the sp2 domain, almost all the functional groups are also removed, leading to weak absorption and poor spectral selectivity again. Herein, we report a new strategy to fabricate large size chemical functionalized monolayer graphene by deep UV irradiation. The layered nano-graphene can be exfoliated to monolayer under short time UV exposure. The new sp2 domain can be restored during long time UV exposure. Furthermore, the edge carbon atom can be activated during UV irradiation, leading several monolayer nano-graphene sheets to aggregate horizontally to form large size monolayer graphene.
Fabrication of Graphene Oxide
Graphene oxide (GO) was synthesized from natural graphite by modifying the Hummer’s method as reported in our previous work . The resulting mixture was washed by 5% HCl solution and DI for dozens of times. Finally, GO solid was obtained after freeze-drying.
Synthesis of Few Layer Nano-Graphene and Growth for Large Size Reduced Graphene Oxide
4.4 mg GO solid was transferred to Teflon-lined autoclave and 12 mL ethanol (or N,N-dimethylformamide (DMF)) was added. The mixture was heated to 176 °C for 5 h. The supernatant was filtered through a 0.22-μm microporous membrane. Finally, the colloidal solution was the few layer nano-graphene solution.
4.4 mg GO solid was transferred to Teflon-lined autoclave and 15 mL DI added. The mixture was heated at 176 °C for 5 h. Then the supernatant was filtered through a 0.22-μm microporous membrane. The colloidal solution was the monolayer nano-graphene solution.
The photoluminescence (PL) and Fourier-transformed infrared (FTIR) were measured on steady-state fluorescence spectrometer (FluoroMax-4, Horiba, Jobin Yvon) and FTIR spectrometer (Nicolet 8700, Thermo Scientific), respectively. The morphology and height were characterized by atomic force microscopy (AFM) operating in tapping mode at room temperature on Si substrate (NT-MDT Prima). The crystallinity of the sample was performed by high resolution transmission electron microscope (HRTEM) (JEM-2100F, JEOL).
Result and Discussion
On the other hand, the FTIR spectra of nano-graphene changes significantly (Fig. 4b) after a long period of UV irradiation (2 h). The first change is that absorption of -COOH at 3150 cm−1 is significantly reduced. At the same time, new C-O-C absorption occurs and it overlaps with previous C-O-C absorption, resulting in extensive C-O-C absorption. Secondly, absorption of C=O moves from 1740 cm−1 to low wavenumber direction (1720 cm−1). This is due to the increase of the conjugate system. The third major change is the appearance of a new C=C in-plane absorption peak at 1562 cm−1. This is because that photo-reduction process of GO can induce deoxygenation and restore the sp2 domain [16, 23, 24]. Finally, a further enhancement of C-H absorption is observed since more H atoms combine with C atoms.
Fluorescence of graphene has been systematically studied. Origin of luminescence is mainly assigned to eigenstate-induced fluorescence caused by the sp2 domain (302–380 nm) and sp3 defect luminescence caused by the oxygen-containing functional group [19, 25, 26, 27]. For the few layer nano-graphene, the Van der Waals (vdW) heterojunction forms due to the stacking of few layers nano-graphene. The vdW heterojunction performs high charge separation. The excited electron in surface defects states induced by oxygen functional groups would transfer to intrinsic states induced by C=C sp2 domain due to the stacking induced band bending. The few layer nano-graphene emit pure UV light. We assign luminescence with peak at 307 nm to the fluorescence of the eigenstate of the sp2 domain. Meanwhile, the visible portion (peak at 500 nm) is derived from the luminescence of the defect states as literature reports [19, 25]. It is clear that fluorescence of the eigenstate of the sp2 domain gradually disappears as the nano-graphene is exfoliated into monolayer and we believe that thickness change of nano-graphene is the main reason for the fluorescence change.
Accompanied with the visible emission change, a surprising phenomenon, UV emission centered at 326 nm with a shoulder at 360 nm is shown and increase with reduction time. The reduction of graphene oxide is usually accompanied by the formation of a new sp2 domain . We speculate that it may be due to the aggregation of nano-graphene causing an increase in the ultraviolet fluorescence.
When we applied k to the Eq. 2, we yield the Gibbs free energy ∆G ≈ − 4.43 kJ/mol. The ∆G < 0 indicates that this reaction is thermodynamics favored at room temperature . Thus, several number of nano-graphene could bond together horizontally to form large size agglomerates. It is worth to point out that large size graphene prepared via CVD owns fine sp2 structure but hardly contains any functional groups for spectral selectivity. Our UV induced nano-graphene aggregation could contain various functional groups which have unique optical property and can be used as identification sites for selective detection.
In summary, we discovered that few layer nano-graphene can be exfoliated to monolayer nano-graphene due to the bonded H atom on the surface under short time deep UV irradiation in protic solution. The monolayer nano-graphene could aggregate into large size monolayer rGO under excess time UV irradiation. The AFM results clearly show that large size monolayer rGO is formed by aggregation of several small nano-graphene sheets. The aggregation of small nano-graphene agrees with the Langmuir adsorption isotherm model, which indicates that the edge of nano-graphene can be activated and can bond with other nano-graphene. This UV-induced growth method may promote the low-cost, large-scale fabrication for monolayer graphene in the future.
This study was funded by the National Nature Science Foundation of China (No. 11674101).
Availability of Data and Materials
All date generated or analyzed during this study are included in this published article.
JQC and JHX conceived the idea. XXH performs the calculations, analyzed the result data, and wrote the manuscript. SJZ and HFP help to analyzed the data. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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