Chemical etching of copper foils for single-layer graphene growth by chemical vapor deposition

Chemical etching of copper foils for single-layer graphene growth by chemical vapor deposition

Accepted Manuscript Frontiers article Chemical Etching of Copper Foils for Single-Layer Graphene Growth by Chemical Vapor Deposition Naoki Yoshihara, ...

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Accepted Manuscript Frontiers article Chemical Etching of Copper Foils for Single-Layer Graphene Growth by Chemical Vapor Deposition Naoki Yoshihara, Masaru Noda PII: DOI: Reference:

S0009-2614(17)30705-4 http://dx.doi.org/10.1016/j.cplett.2017.07.035 CPLETT 34961

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

7 June 2017 12 July 2017 14 July 2017

Please cite this article as: N. Yoshihara, M. Noda, Chemical Etching of Copper Foils for Single-Layer Graphene Growth by Chemical Vapor Deposition, Chemical Physics Letters (2017), doi: http://dx.doi.org/10.1016/j.cplett. 2017.07.035

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Chemical Etching of Copper Foils for Single-Layer Graphene Growth by Chemical Vapor Deposition Author Names: Naoki Yoshiharaa ⃰ and Masaru Nodaa Affiliation:

aDepartment

of Chemical Engineering, Fukuoka University, Fukuoka

814-0180, Japan Corresponding Author E-mail Address: [email protected] Abstract Chemical etching on copper surface is essential as a pre-treatment for single-layer graphene growth by chemical vapor deposition (CVD). Here, we investigated the effect of chemical etching treatment on copper foils for single-layer graphene CVD growth. The chemical etching conditions, such as the type of chemical etchants and the treatment time, were found to strongly influence the graphene domain size. Moreover, a drastic change in the layer structure of graphene sheets, which was attributed to the surface morphology of the etched copper foil, was confirmed by graphene transmittance and Raman mapping measurements. Keywords Chemical etching, Single-layer graphene, Chemical vapor deposition, Copper foil, Surface morphology

1. Introduction Graphene growth on transition metals by chemical vapor deposition (CVD) has attracted significant attention as a practical technique for providing high-quality, large-area samples for applications in numerous fields [1-4]. Among transition metals, copper, owing to its low carbon solubility, has been used to achieve the growth of single-layer graphene [5]. Previous investigations of graphene CVD growth on the copper catalysts have extensively studied the effects of surface morphology, crystallinity, and purity by using deposited membranes or commercially available copper foils [6-14]. Graphene growth on copper depends strongly on the CVD temperature; high CVD temperatures result in the growth of large graphene domains owing to decrease in the nucleation density and an increase in the growth rate. In addition, the surface conditions of a copper catalyst, such as the presence of adsorbed species and morphology, have also been found to affect the graphene CVD growth [9-10]. In particular, oxygen impurities in the copper surface have been shown to decrease the nucleation density and accelerate the graphene CVD growth [11-12]. Therefore, to optimize the graphene CVD process for the growth of the high-quality and large-area graphene, it is crucial to control not only the CVD conditions but also the copper surface conditions. Chemical etching is a necessary pre-treatment for the growth of graphene by CVD on commercially available copper foils because it flattens the surface and removes impurities and the natural copper oxide layer. Although there have been numerous reports of the graphene CVD growth on such etched copper foils [10-14], different chemical etching conditions have been used in each laboratory for the copper surface. Since the chemical etchants used for this treatment greatly influence the etching mechanism and rate of the copper crystal structure, it is essential to understand the influence of chemical etching conditions for the CVD growth of high-quality, large-area graphene. Furthermore, to date, there has been no investigation on the properties of graphene sheets grown on etched copper foils. Here we investigated the influence of chemical etching of copper foils on the CVD growth of single-layer graphene. In this work, the chemical etching treatment was performed for several commercially available copper foils with different crystal structures, by sonication in an iron trichloride (FeCl3) or an ammonium persulfate ((NH4)2S2O8) aqueous solution. These chemical etchants have been known to etch the copper surface by different mechanisms and at different rates. Using these chemical etchants, it is shown that the large differences in the CVD growth and the properties of graphene could be ascribed to a chemical etching effect. We found that the as-grown graphene on the etched copper foils shows considerable changes of the domain size and

density based on the chemical etching conditions, so that the surface morphology of the etched copper foils after the annealing treatment is attributed to graphene nucleation. Moreover, the transparency of graphene sheet grown on the etched copper foils was also investigated to understand the influence of chemical etching on the graphene layer structure. Drastic transmittance change of graphene grown on the copper foils treated with the FeCl3 etchant is demonstrated compared to those treated with the (NH4)2S2O8 etchant, confirming the formation of stacking layer structure in graphene sheets by Raman mapping measurements. Our results show that the chemical etching conditions of the copper foil are also an important factor for the CVD growth of high-quality, large-area graphene, as well as the CVD condition.

2. Experimental 2.1. Chemical etching treatment on copper foils Commercially available copper foils purchased from Alfa Aesar (AA) (purity: 99.8%, thickness: 25 m), Aldrich (AR) (purity: 99.98%, thickness: 25 m), and Nilaco (NC) (purity: 99.9%, thickness: 30 m), were cut to dimension of ~10 mm × 10 mm. The chemical etching of these copper foils was carried out by the immersion of the copper foil into the etchant solution and the application of an ultrasonic wave. FeCl3 (0.2 mol/L) and (NH4)2S2O8 (0.2 mol/L) were used as the chemical etchants. Finally, the etched copper foil was rinsed with distilled water for 3 times. 2.2. Graphene synthesis and transfer Graphene was grown on etched copper foils by ambient pressure CVD at 1000 °C with CH4, H2, and Ar gases. The etched copper foil was set in a quartz tube and heated up to 1000 °C for 40 min under Ar gas. After reaching the CVD temperature, diluted CH4 (20 ppm) and H2 (2.0%) gas were introduced into the quartz tube for 60 min. Eventually, the graphene CVD growth was quenched by stopping the CH 4 supply and then rapidly cooling the sample down to room temperature. Polymethyl methacrylate (PMMA) was spin-coated onto the surface of the graphene and then thermal tape (Revalpha, Nitto Denko) was attached to the PMMA film. Releasing the graphene supported with PMMA and thermal tape, the copper foil was dissolved in an (NH4)2S2O8 aqueous solution (0.2 mol/L). Subsequently, the thermal tape/PMMA/graphene stack was washed with distilled water and transferred onto a target SiO2/Si substrate, and then baked at 120 °C for 15 min to peel off the thermal tape. Finally, PMMA was removed using a hot ethanol aqueous solution, leading the graphene on the SiO2/Si substrate. 2.3 Characterization The crystallinity and crystallographic orientation of the copper foil were measured by X-ray diffraction (XRD, Shimadzu XRD6100). Scanning electron microscopy (SEM, JEOL JSM6060), optical microscopy (Shimadzu OLS4100), and atomic force microscopy (AFM, Bruker Nanoscope V) were used to image the surfaces of the copper foil and graphene. Raman spectra and mapping images of graphene were measured with a Nanofinder 30 (Tokyo Instruments) using 532 nm excitation laser of ca. 1.6 mW. Optical transmittance of the as-transferred graphene sheet on the glass substrate was measured with a UV-VIS spectrometer (Shimadzu UV1800)..

3. Results and discussion First, we performed the X-ray diffraction measurements for several pristine copper foils (see Supporting information, Figures S1a-b). AA copper foils showed a clear diffraction peak at 50.4°, identified as arising from Cu(100) plane, indicating the predominance of these crystalline grains on the AA copper foils. AR copper foils were observed to have three diffraction peaks that were identified as Cu(111) (2=43.3°), Cu(100), and Cu(110) (2=74.1°), revealing a polycrystalline structure. For the NC copper foils, an intense Cu(110) diffraction peak was predominantly present, with the weaker Cu(100) peak also detectable. By contrast, the NC copper foil showed the weak peak originating from the natural copper oxide layer, Cu2O(111) (2=36.5°), as can be seen in Figure S1b [15]. Although this layer may be presented in all copper foils, it was not detected in the XRD profiles of the AA and AR copper foils due to the smaller thickness of the natural copper oxide layer on these copper foils. Other copper oxide layers are not detected in these observations. Figures 1a-c show the SEM images of the graphene domains grown on the AA, AR, and NC copper foils, respectively, at 1000 °C with the flow of 20 ppm CH4, 2.0% H2, and Ar for 60 min after chemical etching with 0.2 mol/L FeCl3 aqueous solution for 15 sec. The hexagonal graphene domains were observed on each of the etched copper foils. Interestingly, the shape of the graphene domain on the NC copper foil did not completely fit, in contrast to the AA and AR etched copper foil which maintained the regular hexagonal shape. To investigate the quality of these graphene by Raman spectroscopy, these graphene domains were transferred onto a SiO2/Si substrate. Figures 1d and 1e show the optical micrograph and the corresponding Raman spectra taken at three different points for the hexagonal graphene domain on the AA etched copper foil. The Raman spectra measured for the graphene domain exhibit two prominent peaks at ~1582 and ~2700 cm–1, corresponding to the G and 2D bands. The relative intensity (I2D/IG) of these bands and the full width at half maximum (FWHM) of the 2D band were ~1.3 and ~37 cm–1, respectively. These data confirm the formation of single-layer graphene [6, 16-17]. Furthermore, Raman mapping measurement also indicated that uniform single-layer graphene comprised most of the area of this hexagonal graphene domain (Figure 1f). Figures 2a and 2b show the etching time evolution of the graphene domain size on the copper foils etched by an FeCl3 or an (NH4)2S2O8 etchant. Here, to investigate the size and density of the graphene domains, the CVD growth of graphene was terminated by stopping the CH4 feedstock prior to covering the entire Cu surface. For the FeCl3 etchant, we found that the average size of the graphene domains grown on each copper

foil increases with increasing etching time. It is noted that despite their identical CVD profiles, the graphene domain sizes on the AA and AR copper foils are greater than that of the graphene on the NC copper foil. On the contrary, the density of graphene domains on the NC copper foil is considerably higher than those of the AA and AR copper foils, as shown in Figure 2c. Interestingly, the graphene CVD growth on the AR etched copper foil treated with (NH4)2S2O8 was not achieved in 1 min, but graphene domains were observed on the copper foil etched for 2 min (Figure S2). This may indicate that graphene nucleation is influenced by the copper surface after the chemical etching treatment. We found that the AA and AR copper foils disappeared in the FeCl3 etchant when the chemical etching treatment is longer than 2 min, while for the NC copper foil such disappearance required 7 min due to the greater thickness. On the other hand, all copper foils treated with the (NH4)2S2O8 etchant could be maintained for the etching times of 10 or more min. This is because the etching rate of copper is different for each chemical etchant. It is noted that the etching rate of the (NH4)2S2O8 etchant was lower than that of FeCl3, but the graphene CVD growth on the copper foil treated with the (NH4)2S2O8 etchant gave larger graphene domains compared to those of the FeCl3 etchant. These results strongly suggested that the chemical etching treatment of the copper foils is advantageous not only due to the removal of a natural copper oxide layer and impurities but also promote favorable CVD growth of graphene on the etched copper foil. To further understand the above results, we compared the surface morphology of pristine copper foils and preheated copper foils after the chemical etching treatment. Here, the preheated copper foils were first heated up to 1075 °C in Ar atmosphere after the chemical etching for 1 min, and then were cooled down rapidly to room temperature. These surfaces correspond to the copper surface prior to the graphene CVD growth. Figures 3a-c show the AFM images of pristine copper foils. The AA copper foil has the relatively smooth surface, although there were many particle-formed protrusions. The surfaces of the AR and NC copper foils widely formed irregularities with the sizes of several tens of nanometers. By contrast, a drastic change was observed for the surface morphology of preheated copper foils after the chemical etching treatment. Although the preheated AA copper surface became smoother than that of the pristine copper foil for each etching treatment, a larger protrusion was observed on the preheated AA copper surface. On the other hand, the AR and NC copper surfaces displayed the step-terrace structures. For the AR copper surface, irregularly distributed step-terrace structure was observed on the copper surface treated with the FeCl3 etchant, while a smooth area is

also confirmed for the surface treated with (NH4)2S2O8. For the NC copper foils, a uniform step-terrace structure is distributed over the entire copper surface for both etchants. This morphology change can be explained by the fact that the chemical etching and the preheating treatment remove the natural copper oxide layer, leading to the reorganization of the copper lattice on these surfaces. We think that these surface morphology change after the preheating process are caused by the chemical etching treatment of the copper foils, resulting in the difference in the as-grown graphene domain size and nucleation density on the different etched copper foils. Thus, the step-terrace structure on the etched copper surface promotes the dissociation of the hydrocarbon feedstocks, leading to the high graphene nucleation density. On the other hand, the comparatively smooth surface morphology decreases the nucleation density, resulting in the formation of the larger hexagonal graphene grains. Next, we compared the surface morphology of the preheated copper foils with and without the chemical etching treatment. Figure S3 shows the AFM images of the preheated copper foils without the chemical etching treatment. For the AA copper foils, it is found that the amount of the fine protrusion was decreased considerably by the chemical etching treatment. Although some of the protrusions remained on the AA copper foils treated with the FeCl3 etchant, no protrusion was observed on the surface treated with (NH4)2S2O8. The AR copper foils also showed a different surface morphology for each chemical etchant. We observed that for the FeCl3 etchant, the step-terrace structure is formed on the entire AR copper surface, while for the (NH4)2S2O8 etchant, this structure is found on only a part of the surface. In contrast to the AA and AR copper foils, no change of surface morphology was observed for the preheated NC copper foils with and without chemical etching. From these results, we conclude that the appropriate combination of the crystal structure of the copper foils and the chemical etching treatment is essential to control the graphene domain size and density due to the surface morphology on the etched copper foils prior to the graphene CVD growth. Transmittance is a significant measure of graphene quality. Here, we investigated the transparency of graphene sheets with dimensions of approximately 10 × 20 mm that were grown on the etched copper foil at 1075 °C for 6 h. Figure 4 shows the transparency at 550 nm wavelength of the graphene sheets transferred onto a glass substrate. More detailed transmittance spectra of graphene sheets are shown in Figure S4 of Supporting Information. For the copper etching time of 1 min (see Figure 4a), with the exception of the AA copper foil etched by the FeCl 3 etchant, our graphene sheets exhibited an average transmittance of 97.4 ± 0.3% for the 550 nm wavelength. These values are consistent with the theoretical absorption values of 2.3% for monolayer

graphene [18]. On the other hand, the transmittance of the graphene sheet grown on the AA copper foil treated with FeCl3 etchant decreased significantly to 94.5%, and was lower than that of bilayer graphene (95.4%). In addition, when the etching time of copper foils was prolonged to 2 min (Figure 4b), the transmittance of the graphene sheets grown on these etched copper surface is almost similar to compare with those for 1 min. Moreover, Raman mapping measurements of the graphene sheet grown on the AA copper foils treated with the FeCl3 etchant is distributed to the lower I2D/IG, indicating a few layers stacking structure of graphene sheet (Figure 5a). Interestingly, the high-quality graphene region that reveals the lower relative intensity of D-band to G-band (ID/IG ~0.15) also showed the graphene sheet grown on the AA copper foils treated by the FeCl3 etchant (Figure S5 of Supplementary contents). This region indicated high uniformity and high quality of the graphene structure [16]. This is suggested that the etching treatment of the AA copper foils with the FeCl 3 etchant is formed on the heterogeneous surface morphology that distributes both of the smooth and roughness area over this etched copper foil. Consequently, the graphene sheet conflated with single-layer and multi-layer graphene was grown on the AA etched copper foil, as discussed below. In contrast, Figures 5b and 5c showed that the majority part of the graphene sheet was formed the single-layer over the AR and NC copper foils. Although the minority part of the multi-layer structure was also presented on these sheets that were grown on the AR and NC copper foils, these were not significantly affected to the transmittance of the graphene sheets. Moreover, for the chemical etching with the (NH4)2S2O8 etchant, the continuous single-layer graphene sheets were grown on all etched copper foils (No data). Finally, we discuss the origin of the stacking layer structure in the graphene sheet observed for the AA copper foil treated with the FeCl3 etchant. The transition metal catalysts such as nickel [19-20] or cobalt [21-22] can easily grow multi-layer graphene by the surface segregation on the cooling process after CVD due to their higher carbon solubility compared to the copper metal. By contrast, due to the low carbon solubility of copper (~0.03 atom% at 1000 °C [23]), this mechanism is not applicable on the copper surface. Rather, two alternative mechanisms have been proposed for the multi-layer graphene growth on the copper surface [24-25]. The first mechanism involves second layer graphene growth on the first formed graphene layer after the first layer graphene is grown over the entire catalyst substrate. The second mechanism involves the formation of the second graphene layer below the graphene domain by the intercalation of carbon radicals dissociated hydrocarbon feedstocks between the first layer graphene and the catalyst substrate [26-28]. Although graphene sheets grown on

each etched copper foils for the transmittance measurement were formed over the entire copper foils using the same CVD profile, the stacking layer structure was confirmed only on the AA copper foil etched with FeCl3. Thus, it is likely that the former stacking layer mechanism is not appropriate for our graphene CVD growth. On the other hand, the latter mechanism may allow for the early stage of the graphene domain growth. Our SEM observations of graphene domains on the AA copper foil treated with FeCl3 often confirmed the presence of a multi-layer graphene domain arranged in an AB stacking configuration (Figure S6). Hence, for the stacking layer mechanism on these AA copper foils, we propose that the second graphene layer grew underneath the first layer due to the intercalation of carbon feedstocks. We also infer that this stacking layer mechanism on the AA copper foil etched with the FeCl3 etchant give rise to the fine protrusion present on the AA copper foils. Further study is underway to characterize the stacking layer growth of graphene on the AA etched copper foils. 4. Conclusion We have investigated the influence of the chemical etching of copper foils for the single-layer graphene CVD growth. Our systematic study indicated that the domain size and the graphene properties were strongly influenced by the combination of the crystal structure of copper foils and the chemical etching treatment. Transmittance measurements indicated that only the graphene sheet grown on the AA copper foil treated with the FeCl3 etchant exhibits values lower than the theoretical value of the single-layer graphene. Moreover, SEM images and Raman spectroscopy measurements elucidated that the stack layer structure is randomly distributed on this graphene sheet. The formation of the observed stacking layer structure is explained by the growth of second graphene layer grew underneath the first layer due to the intercalation of carbon feedstocks. Our work will contribute to the understanding of the single-layer graphene CVD growth on commercially available various copper foils and also to the further development of the graphene-based applications. Acknowledgement The authors acknowledge Mr. A. Adha Sukma, Mr. H. Ji, Mr. M. Izumoto, and Prof. H. Ago of Kyushu University for Raman and AFM measurement help.

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Figure Captions

Figure 1. SEM images of graphene domains grown on (a) AA, (b) AR, and (c) NC copper foils etched with FeCl3 etchant at the CVD temperature of 1000 °C. (d) Optical micrograph of transferred graphene domain from the etched AA copper foil to SiO 2/Si substrate. (e) Raman spectra marked in (d). (f) Corresponding Raman mapping image of (d).

Figure 2. Graphene domain size plotted as a function of etching time with (a) FeCl 3 and (b) (NH4)2S2O8 etchant. (c-h) Optical micrographs of the graphene grown on the etched copper foils treated with (c-e) FeCl3 and (f-h) (NH4)2S2O8 etchant for 1 min.

Figure 3. AFM images of (a-c) the surface of pristine copper foils and preheated copper foils after the chemical etching treatment with (d-f) FeCl3 and (g-i) (NH4)2S2O8 etchant for 1 min.

Figure 4. Transparency of graphene sheets transferred on a glass substrate for 550 nm wavelength. Graphene sheets are grown on the etched copper foils treated for (a) 1 min and (b) 2 min.

Figure 5. Raman mapping images of I2D/IG intensity ratio. (a) AA, (b) AR, and (c) NC copper foils etched with FeCl3 etchant for 1 min.

Graphical abstract

Chemical Etching of Copper Foils for Single-Layer Graphene Growth by Chemical Vapor Deposition Author Names: Naoki Yoshiharaa ⃰ and Masaru Nodaa Affiliation:

aDepartment

of Chemical Engineering, Fukuoka University, Fukuoka

814-0180, Japan Corresponding Author E-mail Address: [email protected] Keywords Chemical etching, Single-layer graphene, Chemical vapor deposition, Copper foil, Surface morphology Highlights 1. Sizes and densities of graphene grown on several etched Cu foils were compared. 2. The surface morphology on the etched Cu foil changed by chemical etchants. 3. The drastic change of the layer structure in the graphene sheets was confirmed.