Self-assembly to monolayer graphene film with high electrical conductivity

Journal of Energy Chemistry 22(2013)52–57

Self-assembly to monolayer graphene film with high electrical conductivity Yi Lua , Xiao-Yu Yanga∗ ,

Bao-Lian Sua,b∗

a. Laboratory of Living Materials at the State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, China; b. Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, B-5000 Namur, Belgium [ Manuscript received January 24, 2013; revised February 1, 2013 ]

Abstract Monolayer chemically converted graphene (CCG) nanosheets can be homogeneously self-assembled onto silicon wafer modified by 3-aminopropyl triethoxysilane (APTES) to form very thin graphene film. The CCG film was characterized by FT-IR, XRD, SEM, TEM and AFM. Results show that CCG sheets formed monolayer film after assembled onto silicon wafer and there is a very tight chemical bond between sheets and wafer. Furthermore, the electrical measurements revealed that the monolayer graphene film has an excellent electrical conductivity. Key words monolayer graphene; self-assemble; electrical conductivity

1. Introduction Graphene, a unique two-dimensional carbon material, has attracted significant interest in their applications as energy materials, especially used in energy conversion/storage systems, because of its high specific surface area, good chemical stability, excellent electrical and thermal conductivity as well as remarkably high mechanical strength. From the view of practical applications, graphene-based film is mostly preferred as electrode material for electrochemical energy storage applications in lithium ion batteries [1−3], supercapacitors [4−6], electrocatalytic [7,8] fuel cell [9−11], and photovoltaic devices [12,13]. It was found that synergetic effects between graphene nanosheets and metal oxides and conductive polymers can be used to develop energy storage materials with high capacity and capacitance, high rate capability and high cyclic performance. For example, graphene/polyaniline composite paper [14,15], graphene/hydrous RuO2 composites [16], graphene-anchored Co3 O4 [17] and graphene-wrapped Fe3 O4 composites [18] for next-generation lithium ion batteries have been investigated in detail [19,20]. Recently, Nirmalraj demonstrated the relationship between the thicknesses of graphene strips and its electrical resistivity, and they showed that when the thickness of graphene strips is increased from a single monolayer to larger thickness,

the resistivity of individual graphene strips is correspondingly increased [21]. Monolayer, chemically converted graphene (m-CCG) is therefore a potential material for graphene thin films which can effectively improve the electrical conductivity of the devices [22−25]. Further researches have focused on the preparation and energy storage explorations of monolayer graphene film materials [26,27]. On the basis of some effective approaches to fabric highly stable electrically conductive multilayer-graphene films (such as spin-coating [28], covalently assemble [29] and deposited [30]), Wang et al. [31] and Becerril [32] have successfully fabricated very thin graphene films with good conductivities by post synthesis process. However, synthesis of monolayer graphene thin films is still challenging, because these graphene films are usually obtained by post-reducing the coating or assembly of graphene oxide sheets on the substrates. Such transfer of the GOs film onto the substrates not only requires very careful control, but also results easily in the poor adhesion of these reduced graphene coatings to the substrates due to the decreasing of the oxygen containing groups and covalent bonds. APTES (3-aminopropyl triethoxysilane) is a familiar organic/inorganic hybrid on which active functional organic groups are existed on both organic and inorganic species. And combining with its well controlled condition of sol-gel hydrolyzation, APTES can be used as a binding agent to

∗

Corresponding authors. Tel: 027-87855322; Fax: 027-87855322; E-mail: [email protected], [email protected] This work was supported by a Chinese Ministry of Education “Changjiang” Innovative Research Team Program (IRT1169), “the Fundamental Research Funds for the Central Universities” (303-47110117, 303-47110118, 2012-yb-04, and 2012-Ia-008), NCET (NCET-11-0688) and RFDP (20110143120006), NSFHB (2011CDB429), NFSC (51101115), and Innovative Research Funds of SKLWUT (2011-la-024, 2012-Ia-008, 2011-PY-2, 2011-PY-3). Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

Journal of Energy Chemistry Vol. 22 No. 1 2013

assemble the graphene sheets onto the silicon surface. Herein, the monolayer graphene film has been synthesized using APTES-modified surface of the device to self-assemble to chemically converted graphene sheets with the residual carboxylic groups. More importantly, the monolayer graphenesupported silicon device shows high electrically conductive. 2. Experimental 2.1. Reagents and materials Graphite powder (D50<400 nm, 99.95%), 3-aminopropyl triethoxysilane (APTES, 99%) and hydrazine monohydrate (99%) were obtained from Aladdin Industrial Corporation. Other reagents were of analytical grade and were purchased from Sinopharm Chemical Reagent Company. All the solutions were prepared with deionized water (>18 MΩ·cm). 2.2. Synthesis of GO and m-CCG nanosheets GO was synthesized from natural graphite powder by the modified Hummers method [33,34]. In a typical procedure, graphite powder was treated with a preoxidation step. The graphite powder (1 g), K2 S2 O8 (0.5 g) and P2 O5 (0.5 g) were put into an 80 ◦ C solution of concentrated H2 SO4 (5 mL). Then it follows the cooling to room temperature overnight. The resulted powder was washed, filtered and dried. The oxidized graphite powder, NaNO3 (1 g) and concentrated H2 SO4 (50 mL) were stirred together in an ice bath. Then KMnO4 (6 g) was added into very slowly. Next, the mixture was transferred to a 35 ◦ C water bath. After stirring for about one hour, 200 mL of water was slowly added into the paste, while the temperature was raised to 95 ◦ C for 30 min. At last, additional 400 mL of water and 20 mL of H2 O2 were added, after which the mixture was filtered and washed. The filter cake was dispersed in water and centrifugated at 8000 rpm for 5 min, and then the supernatant was removed. When repeated several times, the mixture was subjected to dialysis to completely remove metal ions and acids. The final graphite oxide slurry was freeze-dried using a Freeze Dryer FD-1A50. The graphite oxide was exfoliated to generate graphene oxide (GO) followed by centrifugation, a diluted GO colloid solution with a concentration of 0.2 mg/mL was prepared. For chemical reduction [35], GO (50 mL) was loaded in 100-mL round-bottom flask, hydrazine anhydrous (6 μL) and ammonia solution (50 μL) was added in. The flask was put in an oil bath (∼95 ◦ C) for 1 h. The black mixed solution was first isolated by Buchner funnel to remove visible particles, and then filtrate was subjected to dialysis and high-speed centrifugation to obtain stable CCG dispersions. 2.3. Fabrication of graphene-silicon wafer Silicon wafer was treated by ultrasonic washing with

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methanol, acetone and deionized water. Then silicon wafer was hydroxylated by dipping in “piranha” solution (concentrated 98% H2 SO4 : 30% H2 O2 = 7 : 3 ) at 90 ◦ C for 45 min and then washed and dried. APTES was added into an acetone solution with a volume ratio of 1 : 100. Silicon wafer was immersed in the mixture immediately. After 45 min, the silicon wafer was withdrawn and washed by acetone solution and dried. At last, the APTES covered silicon wafer was immersed in a CCG aqueous solution at 80 ◦ C for 10 min. Finally, the covalently assembled graphene-silicon wafer was sonicated to remove small agglomerative particles. 2.4. Characterization Transmission electron microscopy (TEM, JEOL 2100F), scanning electron microscope (SEM, Hitachi S-4800) were used to characterize the size and morphology of the samples. The thickness of the samples was observed by an Atomic force microscope (AFM, Veeco, MultiMode 8) in tapping mode. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained using a Bruker VerTex 80v spectrometer. The X-ray diffraction (XRD) patterns were recorded on a D8 Advance X-ray diffractometer (Bruker) with Cu Kα radiation ˚ Conductivity measurements of graphene thin (λ = 1.5418 A). films were carried out on a Keithley 2400 and a Keithley 2000 using conventional four-probe technique.

Figure 1. SEM image of graphene oxide

3. Results and discussion Figure 1 shows the surface topography of GO sample. After the oxidation process, the graphite oxide was exfoliated to be piecemeal sheets with the size range of 200 nm to 5 μm. Figure 2 shows the typical XRD patterns of graphite, graphite oxide and CCG. After being oxidized, the diffraction peaks, characteristic of graphite, completely disappeared. Instead, a new peak at the 11o (2θ) in pattern (2) appeared, proving that graphite was oxidized. The reappearance of the (002) diffraction line and disappearance of the diffraction peak at 11o (2θ) in pattern (3) give an evidence that graphite oxide

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was reduced to graphene and as such restored the ordered crystal structure. This can also be observed from the TEM image of graphene which dispersed in water. Large area of the graphene sheets are very flat thin layer and some are observed with wrinkled structures in Figure 3(a). Figure 3(b) shows the HRTEM of graphene, large sp2 domains indicate that the ordered crystal structure was mostly restored and GO was chemically reduced to CCG. AFM image of CCG shows that the CCG has a visualized surface morphology. Figure 4 shows single CCG sheets ˚ On the basis of nu[36,37] with an average thickness of 6 A. merous AFM images, CCG sheets can be well dispersed in aqueous solution with single layer.

Figure 2. XRD pattern of graphite, graphite oxide and CCG. (1) Graphite, (2) Graphite oxide, (3) CCG

Figure 3. TEM image of CCG

Figure 4. AFM image of CCG which drop-casting on a freshly cleaved mica surface. The scanning area for (A) is 3×3 μm, the Z range is 0.8 nm; the height of ˚ 7 A, ˚ and 6 A, ˚ respectively (a)–(c) is 6 A,

Journal of Energy Chemistry Vol. 22 No. 1 2013

GO can easily form well-dispersed aqueous solutions because of highly negatively charged surface [38,39]. There are full of epoxy, carboxyl, and hydroxyl groups on the GO sheets. The FT-IR spectra of GO illustrates the presence of C = O at 1725 cm−1 , C–OH at 1409 cm−1 , C–O at 1100 cm−1 , and C–O–C at 1073 cm−1 . After the final reduction with hydrazine, the up-spectrum in Figure 5 shows the absence of the peaks at 1409 cm−1 , 1100 cm−1 , and 1073 cm−1 which indicates that the epoxide and the hydroxyl groups attached to the basal graphene layer have been removed. This observation suggests that the oxygen-containing groups are greatly removed after a reduction process. Whatever, the existence of C = O (1725 cm−1 ) implies that some carboxylic groups are unlikely to be fully reduced, and these groups still exist on the surface of CCG sheets. It is known that both the epoxy group [29] and carboxyl group [40,41] can react with amine group of APTES molecules. So in a previous report [42], GO first attached onto a substrate covered with APTES and then followed by chemical or thermal reduction. However, there is no doubt that the chemical bond O = C–NH probably was destroyed after suffering those reduction processes which can lead graphene to fall off from the wafer. From FT-IR spectra, unreduced carboxylic group on CCG sheets make it possible to directly graft CCG sheets on hydrophilic silicon wafer like GO (Figure 6).

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500 nm and few big sheets can also exist. The overlapping of CCG sheets is observed in Figure 7(a). Notably this phenomenon happened only between big sheets and small sheets. Possibly, this is caused by the non-uniform CCG sheets size. The morphology and thickness of the graphene film was further measured using AFM (Figure 7c). AFM images also show that the graphene film is extremely thin and uniform, which is in good agreement with the SEM observation. In average, the thickness of the CCG sheets on wafer is ca. 1.6 nm, which is much thicker than monolayer graphene (see Figure 4) owing to the presence of organic chains and silicates grafted on silicon wafer, which decreases the flatness of silicon wafer [29,43]. On the basis of the previous research [43], the thickness of 1.6 nm indicates that the film produced here is of monolayer. The electrical conductivity of the graphene thin films was measured under ambient conditions using a standard fourprobe method. 40 nm silver contacts were patterned onto graphene thin films as the electrodes through a shadow mask for a better ohmic contact. Four electrode contacts with an inter-electrode spacing of 5 mm were formed onto the wafer. For each wafer, we choose 10 different positions of the graphene thin film. The calculated average conductivities are range from 300−500 S/cm, and the resistivity of the silicon wafer is ca. 1−5 Ω·cm. Comparing with the conventional post-reducing strategy (about 550 S/cm of graphene thin films) [31,32], our approach not only can be used to prepare very thin monolayer graphene film with high electrical conductivity, but also provide a more simple and effective synthesis concept. In addition, there are covalent bond between CCG sheets and silicon wafer, and therefore the as-prepared graphene thin films have a good stability and can be able to adapt mal-conditions. 4. Conclusions

Figure 5. ATR-FTIR spectra of GO and CCG

The morphology of as-prepared graphene thin film is studied by SEM (Figure 7a and b) and AFM (Figure 7c). SEM images show that the graphene thin film is extremely thin yet uniform and continuous. The size of CCG sheets is around

We report the first example of fabrication of monolayer graphene thin films on silicon wafer via self-assembly of water-processable chemically converted graphene to enhance the electrical performance of the silicon materials. Electrical measurements show a good conductivity and have a very tight binding force between graphene thin films and silicon wafer. We have developed a sample approach to use CCG sheets and the results suggest that we need not further reduction of as-prepared graphene-silicon wafer. The ease of synthesis and

Figure 6. Schematic for the generation of graphene thin films using self-assembly interaction between graphene nanosheets and wafer

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Figure 7. SEM image of CCG (a, b), AFM image of as-prepared graphene-silicon wafer (c)

the exceptional electrical performance of graphene thin film make this route not only have widely applications such as solar cell, display, and optical communication applications, but also have an opportunity for graphene-silicon materials industrialization. Acknowledgements Bao-Lian Su acknowledges Chinese Central Government for an “Expert of the state” position in the frame of “Thousand talents program” and the Chinese Ministry of Education for a “Chang Jiang chair visiting scholar” position at Wuhan University of Technology. Xiao-Yu Yang thanks Hubei Government for a “Chutian scholar” position at Wuhan University of Technology.

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