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Thanh Tam, A Simple Approach to the Fabrication of Graphene-Carbon Nanotube Hybrid Films on Copper Substrate by Chemical Vapor Deposition, Journal of Materials Science & Technology 2015,

A Simple Approach to the Fabrication of Graphene-Carbon Nanotube Hybrid Films on

Copper Substrate by Chemical Vapor Deposition

Nguyen Van Chuc, Cao Thi Thanh, Nguyen Van Tu, Vuong TQ Phuong, Pham Viet

Thang, Ngo Thi Thanh Tam

PII: S1005-0302(15)00070-5

DOI: 10.1016/j.jmst.2014.11.027

Reference: JMST 496

To appear in: Journal of Materials Science & Technology

Received Date: 19 September 2014

Revised Date: 14 November 2014

Accepted Date: 19 November 2014

Please cite this article as: N Van Chuc, C.T Thanh, N Van Tu, V.T Phuong, P.V Thang, N.T Thanh Tam, A Simple Approach to the Fabrication of Graphene-Carbon Nanotube Hybrid Films on Copper

Substrate by Chemical Vapor Deposition, Journal of Materials Science & Technology (2015), doi:

10.1016/j.jmst.2014.11.027.

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A Simple Approach to the Fabrication of Graphene-Carbon Nanotube Hybrid Films on Copper Substrate by Chemical Vapor Deposition

Nguyen Van Chuc1,*, Cao Thi Thanh1, Nguyen Van Tu1, Vuong TQ Phuong2, Pham Viet Thang2, Ngo Thi Thanh Tam1

1

Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam

2

Hanoi University of Science, Vietnam National University, Hanoi, Vietnam

[Received 19 September 2014; Received in revised form 14 November 2014; Accepted 19 November 2014]

* Corresponding author Ph.D.; Tel.: +84 3 7565763; Fax: +84 3 8360705

E-mail address: chucnv@ims.vast.ac.vn (N.V Chuc)

In this study, graphene-carbon nanotube (CNT) hybrid films were directly synthesized on polycrystalline copper (Cu) substrates by themal chemical vapor deposition (CVD) method Graphene films were synthesized on Cu substrate at 1000 oC in mixture of gases: argon (Ar), hydrogen (H 2 ), and methane (CH 4 ) Then, carbon nanotubes (CNTs) were grown uniformly on the surface of graphene/Cu films at 750 oC in mixture of Ar, H2, and acetylene (C2H2) gases Ferric salt FeCl3 solution deposited onto the surface of graphene/Cu substrate by spin coating method was used as precursor for the growth of the CNTs The density and quality of the CNTs on the surface of graphene/Cu films can be controlled by varying the concentration of FeCl 3 salt catalyst

Key words: Graphene; Carbon nanotube (CNT); Hybrid films; Copper substrate;

Chemical vapor deposition (CVD)

Owing to its unique electrical, mechanical and optical properties, graphene with two-dimensional (2D) carbon nanostructure has emerged as a new class of promising materials attractive for various applications, such as transparent electrodes[1–3], field-effect transistors[4,5], supercapacitors[6], composites[7], energy storage materials[8–11], chemical and biosensing[12–15] The combination of 2D graphene of high charge density and one- dimensional (1D) carbon nanotube (CNT) of large surface area generates a flexible three-dimensional (3D)

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graphene-CNT hybrid network with outstanding properties Studies proved that graphene-CNT hybrid nanomaterials exhibit higher electrical conductivities, large specific area and catalytic properties compared with either pristine CNT or graphene[16–18] Graphene-CNT hybrid material has also been applied in many applications including electronics (such as transparent conductors[2,19,20], electron field emitters[21], field effect transistors[20]), supercapacitors[6], Li-ion batters[22,23], sensors[24,25] and biosensors[26–28]

The hybrids were prepared by several approaches including sonication method[16,17], chemical vapor deposition (CVD) method[2,18,29,30], and electrostatic spray technique[31] In these methods, CVD method has emerged as an appropiate approach to synthesize graphene-CNT hybrid materials Different transition metal (e.g, iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), gold (Au), palladium (Pd), platinum (Pt), and ruthenium (Ru)[32,33]) have been used to grow graphene and CNTs Transition metals such as Fe, Co, Ni and Cu are of particular interest, due to their low cost and availability However, Fe, Co and Ni are not preferred for mono or bilayer grephene growth due to their higher-than-desirable capability to decompose hydrocarbons On the other hand, the lower decomposition rate of methane on Cu (since Cu cannot form carbide with carbon thereby resulting in low solubility of carbon in Cu) allows the possibility of controlling the number of graphene layers[34], and wet etchant selectivity to graphene[35] Using a rapid heating and cooling CVD system, Nguyen et al.[2] synthesized thin networks of CNTs with different densities that are controlled by varying the thickness of an iron film sputtered on the graphene/copper substrates However, this method requires the sputtering equipment to produce iron catalyst on the surface of graphene/copper

In this study, we present a simple approach to fabricate graphene-CNT hybrid films on polycrystalline Cu substrate By performing CVD method, graphene layer was synthesized on

Cu substrate in mixture of gases Ar, H2 and CH4 CNTs were subsequently produced on the surface of graphene/Cu film in mixture of gases Ar, H2 and C2H2 The density and quality of the CNTs in the hybrid materials can be controlled by varying the concentration of FeCl3 salt catalyst

2.1 Synthesis of graphene film

The graphene films were synthesized by thermal CVD method of high temperature of

1000 °C in Ar environment (1000 sccm) The polycrystalline Cu with a thickness of 35 µ m and

a size of 1.0 cm × 1.0 cm were used as substrate for graphene-films synthesis process To

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reduce the native copper oxide and to facilitate Cu grain growth on the Cu substrate surface, the samples were annealed at CVD temperature for 30 min in a flow of Ar/hydrogen (1000/ 300 sccm) After 30 min, a flow of methane (CH4, 20 sccm) was introduced for growth process The time for the CVD process was 30 min After a preset graphene growth time, the samples were cooled rapidly under a flow of Ar (1000 sccm)

2.2 Synthesis of graphene-CNT hybrid film

In this work, the ferric salt FeCl3 was used as precursor for the formation of catalytic iron nanoparticles Firstly, it was dissolved in deionized water The resultant solution was subsequently deposited on the graphene/Cu substrate The sample was dried naturally at room temperature to prepare for the growth of CNTs

The CNTs growth process was performed via “fast heating” CVD method, using C2H2 as carbon source The graphene/Cu substrates initially placed outside the heating zone were subsequently transferred into the center of CVD chamber when the temperature of the whole system reached to 750 °C in Ar (30 sccm) This step was carried out by moving the CVD chamber to the proper position The mixture of C2H2 (5 sccm), Ar (30 sccm), and H2 (30 sccm) was simultaneously passed through the tube reactor for 30 min The whole process was finally followed by cooling the CVD system in Ar to room temperature.

In the growth process, catalyst iron nanoparticles play a crucial role in controlling the structure of CNTs As mentioned in many previous articles[2,36,37], it is widely accepted that the diameter, which is the main parameter to determine the quality of CNTs, is defined by the size

of the catalyst nanoparticles For that reason, the concentration of the catalyst precursor and its deposition techniques onto the substrates are the key issues for monitoring the shape and size

of nanoparticles To investigate the influence of the formation of surfactant catalytic iron nanoparticle on the growth density and quality of CNTs, the FeCl3 solution was used with four initial different concentrations (0.001, 0.005, 0.01, and 0.05 mol/L) The deposition process of FeCl3 solution onto the graphene/Cu substrate was conducted via spin coating at 5000 r/min for

1 min

2.3 Sample characterizations

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the samples were obtained by using Hitachi S-4800 and Jeol JEM-1010 instruments, respectively The graphene layer structure was studied using a high resolution transmission

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electron microscopy (HRTEM, FEI TECNAI G20) The concentrations of the chemical elements were determined from the energy dispersive X-ray spectroscopy (EDX, JED-2300 Analysis Station) Raman spectra were accquired using LAMBRAM-1B under ambient condition with excitation laser of He–Ne (wavelength: 632.8 nm) Atomic force microscopy (AFM) image was accquired in the tapping mode using Agilent PicoScan 2500

3 Results and Discussion

Fig 1(a) and (b) shows the photographs of Cu substrate (with area of 1.0 cm × 1.0 cm) before and after graphene growth, respectively Homogeneous growth of graphene film all over the area was observed Luster of the Cu substrate was changed to grayish shade after graphene growth Graphene on Cu was grown by surface adsorption process[35] Fig 1(c) shows a typical SEM image of Cu substrate surface after CVD process The white lines observed on graphene film (Fig 1(c)) are graphene wrinkles The wrinkled feature of the graphene films is believed

to be due to accommodation of the differences in the thermal expansion coefficients between graphene or graphite and Cu substrate (αgraphene = –6 × 10–6/K at 27 °C; αgraphite = 0.9 × 10–6/K between 600–800 °C; αCu = 24 × 10–6/K[38]) This is indicative of graphene continuity, since these wrinkles span the Cu grain boundaries[39,40] Fig 1(d) shows the typical EDX characterization of the graphene film grown on the surface of Cu substrate The calculated results from the EDX characterizations measured at four different points (with each point area

of 0.6 mm × 0.6 mm) showed that the concentration of C and Cu were 2.91 ± 0.37 and 97.07 ± 1.35 wt% , respectively

To estimate clearly the thickness of the graphene film, we transferred the graphene film from the Cu substrate to SiO2 substrate Fig 2(a) shows the AFM image of the graphene film after transferring from the Cu substrate to SiO2 substrate A uniform color contrast in the AFM image indicates uniform graphene thicknesses, although bright lines in the graphene films were formed during the transfer process and due to the difference of thermal expansion between graphene and copper during CVD process The height profiles of the cross-section lines associated with the AFM images were used to indicate the thickness of the graphene films AFM image indicates that the thickness of the graphene film is about 1 nm

High-resolution transmission electron microscopy (HRTEM) image can provide direct evidence of the number of graphene layers Fig 2(b) indicates an HRTEM image of the graphene film after transferring from the Cu substrate to a copper grid for TEM examination

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HRTEM image indicates that the number of graphene layers is two layers (0.68 nm thick) This result is consistent with the thickness of the transferred film as analyzed by AFM

Raman spectroscopy is a powerful, yet relatively simple method to characterize the thickness and crystalline quality of graphene layers Raman spectroscopy was performed under excitation laser (λ = 632.8 nm) on the CVD grown graphene Raman spectrum of graphene film (Fig 2(c)) reveals that the CVD-grown graphene films exhibit a graphitic G at 1580 cm–1 and second order resonance double-resonance 2D peaks at 2710 cm–1, whilst there is no apparent

defect related to D peak in the synthesized graphene film It is known that I2D/IG is a sensitive probe of the number of graphene layers[39,41] The ratio I2D/IG of about 2–3 is for monolayer

graphene, 2>I2D/IG>1 for bilayer and I2D/IG<1 for multilayers[39,42] Based on ratio I2D/IG = 1.18

we can conclude that the graphene film grown on the Cu substrate contains more than one layer

Beside the exceptional properties that contribute to the overall characteristic of hybrid material, graphene has an important role in the growth of CNTs It is considered as a barrier that prevents the diffusion of catalytic iron particles formed under high temperature condition

of CVD process into the copper substrate that could lead to the deactivation of the iron catalyst[2] To evaluate clearly the role of graphene during growth, CNTs were synthesized on two different copper substrates (with and without graphene layers on the surface) under the same CVD conditions Structural properties of the bare Cu substrate and graphene-coated Cu substrate after the growth of CNTs were analyzed by SEM As shown in Fig 3(a), the sample without graphene is partly coated by a thin network of CNTs with large diameter Moreover, a certain amount of amorphous carbon deposited around the CNTs can be observed either Meanwhile, after growth, the higher density of CNTs with high purity covered uniformly the entire surface of graphene-coated Cu substrate (Fig 3(b))

Fig 4(a–d) shows typical FESEM images obtained from graphene/copper substrates coated with concentration of FeCl3: 0.001, 0.005, 0.01, and 0.05 mol/L after the CNT growth process, respectively With low concentration of FeCl3 (0.001 mol/L), a thin network of CNTs

is distributed non-uniformly on the graphene surface, as shown in Fig 4(a) As the concentration of FeCl3 is increased to 0.005 and 0.01 mol/L, the graphene surfaces are covered

by a denser CNT network due to the provision of more catalyzing nanoparticles, as indicated in Fig 4(b) and (c), respectively The diameter of CNTs is uniform and there are no amorphous carbon deposited around the CNTs However, when the substrate is coated with higher concentration of FeCl3 (0.05 mol/L), a certain amount of amorphous carbon deposited around

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the CNTs can also be observed The Fe nanoparticles are probably difficult to form due to the cohesive agglomeration That is why we have observed low density and large diameter CNTs,

as indicated in Fig 4(d) Insets in Fig 4(a–d) show their corresponding high resolution FESEM images where the quality of CNTs can be observed clearly

Comparison of the CNT Raman spectrum shown in Fig 5(a) to the graphene spectrum (Fig 2(b)) clearly indicates the presence of CNTs with the appearance of D peak at ~ 1376

cm–1, and the broadening and decreased intensity of the 2D peak at ~ 2713 cm–1 To observe clearly the morphology, quality of the graphene-CNTs hybrid film, the sample was transferred from Cu substrate onto Ni grid for TEM analyst Fig 5(b) indicates the typical TEM image of graphene-CNTs hybrid film grown with concentration of FeCl3 solution of 0.01 mol/L The TEM image demonstrates that grain boundaries of the graphene films are continuous The CNTs exhibits bamboo-like structure The diameter of CNTs is about 30–40 nm

By “fast-heating” CVD technique in atmospheric pressure, the CNTs network was synthesized onto the surface of the graphene/copper substrate The quality and density of the CNTs were controlled via the concentration of FeCl3 catalyst Optimized FeCl3 catalyst concentration for the growth of CNTs on the surface of graphene/copper was 0.01mol/L The CNTs synthesized had bamboo-like structure and uniform diameters in the range of 30–40 nm These results open highly applicable for using graphene-CNTs hybrid film in electrochemical biosensor and field effect transistor biosensor

Acknowledgements

Funding of this work was supported mainly by the National Foundation for Science and Technology Development (No 103.99-2012.15) A part of work was supported by VAST 03.06/14-15 Besides, a part of the work was done with the help about devices of the IMS Key Lab

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Figure captions

Fig 1 Photographs of Cu substrate before (a) and after (b) CVD process, typical SEM image (c)

and EDX spectrum (d) of Cu substrate after CVD process

Fig 2 (a) AFM image of graphene layer after transferring from the Cu substrate to SiO2 substrate, (b) HRTEM image of graphene layer after transferring from the Cu substrate to copper grid and (c) Raman

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Fig 3 SEM images of CNTs grown onto the surface of: (a) Cu substrate, (b) graphene/Cu

substrate Insets in (a) and (b) are their corresponding TEM images

(a) 0.001 mol/L, (b) 0.005 mol/L, (c) 0.01 mol/L and (d) 0.05 mol/L Insets in (a–d) are their corresponding images at high magnifications

Fig 5 (a) Raman spectrum and (b) TEM image of graphene-CNTs hybrid film grown with

concentration of FeCl3 solution of 0.01 mol/L Inset in (b) is its corresponding TEM image

Figure list

Fig 1