Polymer Adsorption on Graphite and CVD Graphene Surfaces Studied

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Polymer Adsorption on Graphite and CVD Graphene Surfaces Studied by Surface-Specific Vibrational Spectroscopy Yudan Su, Hui-Ling Han, Qun Cai, Qiong Wu, mingxiu xie, Daoyong Chen, Baisong Geng, Yuanbo Zhang, Feng Wang, Y. Ron Shen, and Chuanshan Tian Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02025 • Publication Date (Web): 14 Sep 2015 Downloaded from http://pubs.acs.org on September 16, 2015

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Polymer Adsorption on Graphite and CVD Graphene Surfaces Studied by Surface-Specific Vibrational Spectroscopy Yudan Su1,4,†, Hui-Ling Han2,†, Qun Cai1, Qiong Wu1,4, Mingxiu Xie3, Daoyong Chen3, Baisong Geng2,5, Yuanbo Zhang1,4, Feng Wang2, Y.R. Shen1,2,*, Chuanshan Tian1,4,* 1

State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China 2

Department of Physics, University of California, Berkeley, CA 94720, United States 3

State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, Shanghai, China.

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Collaborative Innovation Center of Advanced Microstructures, Fudan University, Shanghai 200433, China

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School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China

ACS Paragon Plus Environment

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Table of Contents Graphic

Keywords: CVD Graphene; Graphite; Surface residue; Self-assembled alkane; Surface structure; Sum frequency vibrational spectroscopy.


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ABSTRACT: Sum-frequency vibrational spectroscopy was employed to probe polymer contaminants on CVD graphene and to study alkane and polyethylene (PE) adsorption on graphite. In comparing the spectra from the two surfaces, it was found that the contaminants on CVD graphene must be long-chain alkane or PE-like molecules. PE adsorption from solution on the honeycomb surface results in a self-assembled ordered monolayer with the C-C skeleton plane perpendicular to the surface and an adsorption free energy of ~42 kJ/mole for PE(H(CH2CH2)nH) with n~60. Such large adsorption energy is responsible for the easy contamination of CVD graphene by impurity in the polymer during standard transfer processes. Contamination can be minimized with the use of purified polymers free of PE-like impurities.


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Graphene prepared by chemical vapor deposition (CVD) has generated tremendous interest for its potential applications in electronics1, energy storage2 and surface coating3. One of the major difficulties in preparing high-quality CVD samples is to obtain pristine graphene free of contaminants4. The popular method for transferring CVD graphene from metal foil onto a substrate involves coating of a polymer film, such as poly(methyl methacrylate) (PMMA, [CH2C(CH3)(CO2CH3)]n) and polystyrene (PS, [CH2CH(C6H5)]n), on graphene grown on metal foil. The polymer can then be used to hold graphene when the metal is etched away and support the graphene in the ensuing transferring process1. Although the polymer is highly soluble in solvent and should be easily removed from graphene, it has been found, by AFM 5,6, STM 7, and XPS5,8, that transferred CVD graphene inevitably has organic residues on its surface that are difficult to remove. The surface contaminants can affect mechanical9, thermal10, electrical properties

5,6,11,12

and wettability13 of graphene and hamper its applications. Why graphene is

easily contaminated in the transferring process and what are the organic residues are still a puzzle, although recent works suggest that CH2 groups like to adsorb on graphene13,14. Here, we have launched a study using surface-specific sum-frequency vibrational spectroscopy (SFVS)15 to probe organic species adsorbed on surfaces of CVD graphene and cleaved graphite. We found that the vibrational spectra of transferred CVD graphene were dominated by CH2 stretching modes, closely resembling the spectra of polyethylene (PE, (H(CH2CH2)nH) on graphite (See Figure 1a and b). This observation indicates that the organic contaminants on CVD graphene are not residues of PMMA or PS, but likely from long-chain alkane or PE-like molecules. From the polarization dependence of SFVS, we find that the C-C skeleton plane of adsorbed PE or PE-like molecules is perpendicular to the graphene or graphite surface. Apparently, adsorption of PE or PE-like molecules is via interaction of half of their methylene


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groups with the surface. As can be detected in the vibrational spectrum, such CH2 groups have their stretch frequency red-shifted by 10 cm-1 with respect to those pointing away from the surface. The adsorption isotherm deduced from SFVS measurement on graphite allows determination of the Gibbs adsorption energy (∆G) for specific PE. It was found that ∆G for PE (H(CH2CH2)nH) with n~60 adsorbed on graphite from chloroform solution is -42 kJ/mol. The large adsorption energy is the cause of strong adsorption of long alkane-chain molecules or PElike molecules on graphene and graphite from solution. The SFVS setup was similar to the one described earlier16, 17. A picosecond Nd:YAG (Ekspla) laser with 20 Hz repetition rate was used to generate a visible beam at 532 nm and a tunable IR beam between 2700 and 3200 cm-1. The two beams overlapping on a sample had pulse energies and beam spot sizes of 140 µJ and 1.7 mm for the visible and 30 µJ and 1.0 mm for the IR, respectively. As described elsewhere in detail16, the SF output intensity, after data analysis to remove geometric factors and Fresnel coefficients, yields amplitude of the surface nonlinear t susceptibility, χ s(2) , that characterizes the surface. Phase-sensitive SFVS further allows t deduction of Im χ s(2) that directly characterizes surface vibrational resonances including the net t polar orientation of the contributing molecular moieties18. In the case of discrete resonances, χ s(2) t and Im χ s(2) can be written as

t (2)

t (2)

χ s = χ NR + ∑ q

t (2)

Im χ s = ∑ q

t Aq

ωIR − ωq + iΓ q t − Aq Γ q

(ωIR − ωq ) 2 + Γ 2q


(1)

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t t (2) Here, χ NR denotes the non-resonant contribution, and Aq , ωq and Γ q represent the amplitude, frequency, and damping constant of the qth vibrational resonance, respectively. The nonvanishing t elements of χ s(2) provide information on the orientation of the molecular moieties contributing to

the resonances. Commercial PMMA (molecular weight 95000) in anisole solution (Micro-Chem Inc.) and PS (molecular weight 97400, Sigma-Aldrich) solution were used to coat CVD graphene grown on a copper foil and, after the copper was etched away, to transfer the graphene to a fused silica substrate. The polymer was then dissolved off in acetone and chloroform for PMMA and PS at 50 oC, respectively. To have a graphite surface to compare with the CVD graphene surface, the same procedure of coating and removal of PMMA or PS was followed on a freshly cleaved HOPG graphite surface. Specific PE or long-chain alkane adsorption on a freshly cleaved graphite surface was investigated on samples prepared by immersing the graphite in chloroform solution of PE (H(CH2CH2)nH, n~60, Sigma-Aldrich) or dotriacontance (CH3(CH2)30CH3, 97%, Sigma-Aldrich). The sample was then taken out of the solution, rinsed with pure chloroform for 5 seconds, dried and measured by SFVS in air and scanning tunneling microscopy (STM) in vacuum. Because detectable desorption of such adsorbed molecules in chloroform occurs on the time scale of minutes, surface coverage of the adsorbed molecules on graphite so prepared remains the same as that in solution. In the entire process of sample preparation, only glassware was used as container which was soaked in H2SO4(98%)+NoCromix for 1 day, thoroughly rinsed with ultrapure water (18.2 MΩ·cm), then dried by heating. To test purity of polymer and organic solvents, a freshly cleaved graphite surface was dipped in the solutions, then taken out and measured with SFVS to make sure no CH feature on the honeycomb surface.


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t 2 Figure 1. The ssp χ S(2) spectra of CVD graphene after removal of (a) the PMMA coating and t (b) the PS coating on it. The ssp χ S(2)

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spectra of (c) air/PMMA and (d) air/PS interfaces,

respectively. Label s- in the figures denotes symmetric vibrational mode. Figure 1c and d display the ssp (s-, s-, and p-polarized SF, visible, and IR beams, respectively) t

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χ S(2),ssp spectra of air/PMMA and air/PS film interfaces in the CH stretching region. They are essentially the same as those reported in the literature19, 20. The vibrational resonances at 2955 cm-1 for PMMA and 3070 cm-1 for PS are from -OCH3 and aromatic CH stretches, respectively. After removing the polymer film from the CVD graphene surface in a proper solution, SFVS is used to detect organic residues on graphene. Their χ S(2),ssp

2

spectra of the CVD graphene surface

with PMMA and PS removed are presented in Figure 1a and b, respectively. They still exhibit


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prominent features in the CH stretching range, indicating hydrocarbon residues present at the surface, but they are very different from the spectra of PMMA and PS in Figure 1c and d. There is no longer detectable trace of OCH3 stretching modes from PMMA or aromatic CH stretching modes from PS. Apparently, PMMA and PS on graphene were effectively removed, but some foreign hydrocarbon contaminants were left on the surface in the process. The spectra resemble that of the air/PE surface reported in previous study21 as well as that of a PE monolayer adsorbed on graphite presented in Figure 2a, which shows a strong peak at 2850 cm-1 attributable to the symmetric CH2 stretching mode (d+ mode) and some weak features between 2850 and 2950 cm1

originating from several overlapping CH stretching modes (to be discussed later). FTIR

spectroscopy on the same samples also reveals dominant features of CH2 stretches (presented in Figure S1 of the Supporting Information (SI)). The result here suggests that the contaminants on graphene are likely long alkanes or PE-like molecules, which may exist as impurities in commercial PMMA and PS (as well as solvents being used) and are known to have low solubility in organic solvent. To facilitate further investigation of adsorbates on graphene, we recognized that a clean graphene surface should be essentially the same as a freshly cleaved graphite surface. To prove this, we subjected a freshly cleaved graphite surface through the same polymer coating and removal procedure used to transfer CVD graphene, and did the SFVS measurement on it at the end. The observed χ S(2),ssp

2

spectrum, depicted in Figure S2 in SI, is indeed very much the same

as those in Figure 1a and b, showing the same contaminants appearing at the surface. We can then conveniently use graphite to represent graphene in studies of molecular adsorption on graphene. Long-chain alkanes or PE-like molecules are known to readily adsorb on graphite from earlier STM studies because of the close match between the C-C skeleton of alkanes and


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the honeycomb lattice of graphite surface.25 However, the adsorption kinetics has not yet been studied and the adsorption geometry of alkanes is still not certain. We have carried out a SFVS study of PE (H(CH2CH2)nH with n ~60) and alkane (CH3(CH2)30CH3) adsorbed from chloroform on a freshly cleaved graphite surface. As seen in Figure 2a, their χ S(2),ssp

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spectra are nearly the

same. They are also close to those in Figure 1a and b describing the contaminated graphene surface. Since it is known from STM imaging that alkanes adsorbed on graphite from solution form a self-assembled ordered monolayer (as shown in Figure 3b and c), and the spectral intensities of Figure 2a are comparable to those in Figure 1a and b, we can conclude that the surface coverage of contaminants on the CVD graphene is of the order of a monolayer.


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Figure 2. The χ S(2),ssp

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spectra of (a) PE and dotriacontane (CH3(CH2)30CH3) adsorption on

graphite surface, respectively, and (b) PS transferred CVD graphene. The Im χ S(2),ssp spectra of (c) monolayer PE molecules adsorbed on graphite surface and (d) PS transferred CVD graphene. The scattered points in (a)-(d) refer to the experimental data and the solid lines are the fitting curve using Eq.(1) with 7 discrete modes. (e) The 7 discrete modes obtained from global fitting of the χ S(2),ssp

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spectrum in (a) and the Im χ S(2),ssp spectrum in (c). (f) A schematic cartoon showing

molecular configuration of PE on the graphite/graphene surface. To better resolve the resonant features of the SF vibrational spectra and understand the molecular configuration of PE on graphite and CVD graphene, phase-sensitive SFVS was used. t The observed Im χ S(2),ssp spectra of PE on graphite and CVD graphene are presented in Figure 2c

and d, respectively. Quantitative analysis was carried out on the spectrum of PE on graphite by 2

decomposing the SF spectra into discrete modes. With both χ S(2),ssp and Im χ S(2),ssp spectra in hand, unique decomposition of the spectra into 7 discrete modes by fitting of the spectra using Eq. (1) can be achieved, as described in Figure 2e with the fitting parameters listed in Table 1. The dominating feature is the two oppositely orientated d+ modes of CH2 stretch at 2840 cm-1 and 2850 cm-1. These two modes, also existing in the Im χ S(2),ssp spectrum of graphene in Figure 2d, can be attributed to two different CH2 groups, one pointing toward the surface and the other away from the surface. The negative mode corresponds to the groups facing the surface. It is redshifted by 10 cm-1 because of van der Waals interaction of CH2 with the carbon honeycomb surface. Associated with these d+ modes of CH2 are the Fermi-resonance (FR) modes appearing at 2912 cm-1 and 2930 cm-1 also with opposite signs21. The modes at 2873 cm-1, 2892 cm-1 and


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2945 cm-1 can be assigned to s-CH3, CH, and FR-CH3, respectively18, because the PE unavoidably contains defects with CH3 and CH groups in side branches which is also visible in the FTIR spectrum of PE (Figure S3). In previous STM studies of alkanes on graphite, there exist debates on whether the C-C skeleton plane is perpendicular or parallel to the graphite surface14,22,23,24. Our assertion of the perpendicular geometry (illustrated in Figure 2f) based on the SF spectroscopic observation is further supported by the polarization dependence of the SF spectra (shown in Figure S4a and b and discussed in the SI). It is also consistent with the observed strong intensity of the d+ mode in Figure 1a and b.

Table 1. Global fitting parameters for the χ S(2),ssp

2

and Im χ S(2),ssp spectra of PE adsorbed on

graphite. Label s- and F.R.- denote symmetric vibration and Fermi resonance, respectively. ωq/(2πc) (cm-1)

2840

2850

2873

2893

2912

2930

2945

Mode

s-CH2,down

s-CH2,up

s-CH3

CH

F.R.-CH2,down

F.R.-CH2,up

F.R.-CH3

Γq/(2πc) (cm-1)

8.79

8.72

12.00

8.16

7.93

9.41

13.97

Amplitude

-3.81

2.23

0.72

-0.49

-0.99

0.89

0.84

To verify the surface morphology of alkanes or PE on graphite is the same as those in early STM studies, we took an STM image of our sample. For better surface quality, we chose dotriacontane (CH3(CH2)30CH3), instead of PE (H(CH2CH2)nH with n~60), on graphite because the unavoidable side branches of PE would lead to larger surface roughness and poorer images, while their SF spectra are the same (see Figure 2a and Fig. 3a). The obtained image is shown in


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Figure 3b, with a magnified version in Figure 3c. It is shown in Figure 3b that the alkane molecules form a well-ordered monolayer. The length between two parallel boundaries (white lines in Figure 3b) is about 4.2 nm, matching nicely with the length of all-trans dotriacontane molecules25. In combining with the above-discussed SF spectral analysis, our results suggest that C-C skeleton of the self-assembled alkane is perpendicular to the graphite surface in air26.

Figure 3. (a) The χ S(2),ssp

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spectrum of dotriacontane (CH3(CH2)30CH3) molecules on graphite.

(b) STM image of dotriacontane molecules on graphite. The self-assembled molecules lying on the surface appear as in orderly aligned structure. (c) Magnified STM image of (b).


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Figure 4. (a) The χ S(2),ssp spectra of PE (H(CH2CH2)nH with n~60) on graphite prepared in solutions with different concentrations. (b) The adsorption isotherm of PE at the chloroform/graphite interface deduced from the spectra in (a). The dashed line indicates the initial slope of the adsorption isotherm. To understand why long alkanes adsorb strongly on graphite and graphene, we used SFVS to measure the Gibbs adsorption energy of PE adsorbed on graphite from chloroform at room 2

temperature. The χ S(2),ssp spectra of PE (H(CH2CH2)nH with n~60) on freshly cleaved graphite prepared in chloroform with different PE concentrations are displayed in Figure 4a. The spectral profile appears unchanged with different bulk PE concentration, suggesting that the configuration of adsorbed PE is the same, as expected from the adsorption geometry discussed earlier. The relative surface coverage of PE, Θ ≡ N s / N s ,sat , is then proportional to the amplitude

of the d+ mode of alkane at 2840 cm-1 or 2850 cm-1, and is plotted in Figure 4b as a function of bulk PE concentration. Here, Ns,sat is the saturated surface density. Note that at 6.6×10-8 molar fraction of PE, corresponding to 820 nM, the surface coverage already appears saturated. This


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indicates a very large adsorption energy (∆G) of PE on graphite from chloroform. Quantitatively, we can deduce ∆G following the simple Langmuir model with the relative surface coverage Θ given by Θ = x / ( x + e−∆G / RT ) , where x is the bulk concentration of PE in molar fraction. From

the initial slope of the adsorption isotherm in Figure 4b, we find ∆G = -42kJ/mol for PE of n~60 adsorbed on graphite from chloroform solution. Such a large adsorption energy can be attributed to the large number of CH2 units interacting with the matched honeycomb lattice structure of the graphite surface and small solubility of long alkane in chloroform (and most of organic solvents). Longer PE is expected to have even larger adsorption energy and lower solubility in solvent. Thus, a tiny amount of PE-like impurity in polymers or solvents used for transferring of CVD graphene can easily result in high surface contamination that can hardly be removed by organic solvents27.

Figure 5. The χ S(2),ssp

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spectrum (red) of high-purity-PS-transferred CVD graphene on a fused

silica substrate. The spectrum of commercial-PS-transferred CVD graphene (black) is shown for comparison.


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Realizing that surface contamination of polymer-transferred CVD graphene is likely from alkane or PE-like impurities in commercial polymers, we synthesized PS with special care to avoid alkane or PE-like contaminant. We also made sure that the solvent used did not contain PE-like impurities. Indeed, using our PS instead of the commercial polymers in treating the graphite and graphene samples, we found no detectable trace of alkane or PE modes in the SFVS 2

spectra. Figure 5 shows the comparison of the χ S(2),ssp spectra from two polymer-transferred CVD graphene samples, one prepared with the commercial PS and the other with our own synthesized PS. The CH stretching modes from PE-like molecules are completely suppressed in the latter. In conclusion, we have clearly demonstrated here that the surface of graphene is sensitive to long alkane or PE-like impurity. Contamination can easily happen if no special caring of alkane or PE-like impurity is taken in the transferring of CVD graphene, and can be avoided if purified polymers are used. Such clean CVD graphene free of polymer contaminants should be highly desired in many applications.

ACKNOWLEDGMENT Y.D.S. and C.S.T. acknowledge financial support by the NSFC (No.11374064, No.11290161, and No. 11104034) and NCET (No.130141). Q.W. and Y.Z. acknowledge financial support from the National Basic Research Program of China (973 Program; grant Nos. 2011CB921802 and 2013CB921902), and NSFC (grant no. 11425415).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected] Author Contributions †Y.D.S and H.L.H. contributed equally to this work.

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SFG

VIS

0.2

IR

Imχ(2) S,ssp (arb. unit)

ω IR

ω VIS

10 cm-1

0.0

-0.2

ACS Paragon Plus Environment -0.4

2800

2850

2900

Wavenumber (cm-1)

2950

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

(a)

Nano Letters (b)

(c)

(d)

Page 20 of 24


Page 21 of 24

Nano Letters

PE or alkane on graphite 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

PS transferred CVD graphene

(a)

(b)

(c)

(d)

(e)

(f)


Nano Letters (a)

1 2 3 4 5 6 7 8 9 10 11 12 13

Page 22 of 24

(b)

10nm (c)


Page 23 of 24 (a)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Nano Letters (b)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33


Page 24 of 24

Polymer Adsorption on Graphite and CVD Graphene Surfaces Studied

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