In Situ Functionalization of Graphene with Reactive End Group

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In-Situ Functionalization of Graphene with Reactive End Group Through Amine Diazotization Md Zakir Hossain, and Natsuhiko Shimizu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08454 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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The Journal of Physical Chemistry

In-situ Functionalization of Graphene with Reactive End Group Through Amine Diazotization Md. Zakir Hossain* and Natsuhiko Shimizu

International Research and Education Center for Element Science, Graduate School of Science and Technology, Gunma University, Kiryu City, Gunma 376-8515, Japan.

*Corresponding author: E-mail: [email protected]; Tel. +81-277-30-1625

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ABSTRACT Functionalization of graphene with active functional group such as thiol is higly desirable for application in highly sensitive graphene based devices including chamical and bio-sensors. Here we report a powerful and common chemical method for functionalization of graphene with varieties of active functional groups. Instead of using pre synthesized stable diazonium salts, the reactant amine is used for graphene functionalization, which gives much more flexibily to functionalize graphene with desired end group. Using two different molecules, 2-aminoethanethiol (HS-C2H4-NH2) and 3,5-difluorophenylamine (F2C6H3-NH2), we found that amine derived diazonium salts undergo spontaneous in-situ reduction on graphene preloaded into the reaction tube resulting into functionalized graphene with organically active functional species HS-C2H4 and F2C6H3. The pristine and modified epitaxial graphene (EG) on SiC are characterized using x-ray photoelectron spectroscopy (XPS), Raman and scanning tunneling microscopy/spectroscopy (STM/STS) at room temperature. The present functionalization method can be utilized to functionalize graphene with varieties of active groups since there are huge number of functional amines commercially available.

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INTRODUCTION Unique and extraordinary physical, chemical and electronic properties make graphene as the most promising two-dimensional materials for the next generation technologies.1-3 This two dimensional material is expected to be utilized in various applications such as ultra-sensitive sensor, high-frequency transistors, photonics and optoelectronics devices such as displays, touch screens, light-emitting diodes and solar cells, etc.1-5 However, to realize such graphene-based devices, we have to overcome some real challenges in controlling the graphene’s

properties and its integration into the practical devices.6-8 For example, because

of the chemical inertness of graphene towards molecules, the pristine graphene cannot be utilized in various types of sensor such as chemical and bio-sensors, where the graphene based electrode needs to interact with desired target molecules. Indeed, the pristine graphene need to modify to make it suitable for many of those next generation electronics and other applications. One of the most widely used techniques to tune material’s properties is the chemical method.6-7 However, unlike silicon and other materials, bonding nature of sp2 C atoms in graphene make it extremely low chemical reactivity towards molecule. Hence one of the real challenge for tailoring the graphene’s properties leading to the development of graphene based device and new graphene derivatives is to develop a versatile chemical approach that can be utilized for functionalization of graphene with varieties of active functional groups. To date a number of studies have been reported on the chemical modification of graphene. Both inorganic and organic approaches were made for covalent binding of atoms or molecules onto the graphene surface. Among inorganic approaches, oxidation, hydrogenation, and halogenations are prominent. Recent studies on the covalent modification of graphene indicate that only the reactive radical species such as CH3, Cl, O and H undergo facile reaction with graphene C atoms and form covalent bonding.9-14 However, the respective source molecules for those radicals are very limited in numbers, hence, the method cannot be utilized for desired functionalization of graphene. One of the prominent and well studied chemical methods for modification of graphene and other carbon materials is the reduction of diazonium compounds 3 ACS Paragon Plus Environment


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onto the materials surfaces.12,15-20 Binding of molecules onto the material surface using diazonium salts is usually done through electrochemical process or simply dipping the sample into the diazonium salt solution in organic solvents.12,15,17,20 Indeed the surface reduction process of diazonium salt also involves the creation of radical species that ultimately bind onto the surface as schematically shown in figure 1(a).12,15 In previous studies, either commercially available or freshly synthesized and purified diazonium salts are used. Usually diazonium salts are synthesized through well-known diazotization of primary amine as shown in figure 1(b). However, commercially available diazonium salts are limited because diazonium salts freshly synthesized specially from apliphatic amines are not always stable enough to isolate from the reaction mixture. Hence, in spite of number of studies attempting to functionalize graphene, the functionalization of well defined pristine graphene with active functional group (such as –SH and COOH) that can be tailored to bioactive surface through further modification could not achieved yet. Recently Min et al. have demonstrated that vertically aligned chemisorbed molecules does not change the major charge carrier of reduced graphene oxide and transferred graphene channel field effect transistor compared to horizontally aligned physisorbed molecules.20 Hence the graphene functionalized with active functional group holds the great promise for development of ultra-sensitive biosensors. A large numbers of different amines are commercially available that do not result stable diazonium salts through diazotization reaction. In this study, we focus on the functionalization of epitaxial graphene (EG) on SiC with reactive end group such as –SH through in-situ reduction of amine derived diazonium salts instead of using pre-synthesized salts. Using two different

molecules,

2-aminoethanetiol

(HS-C2H4-NH2)

and

3,5-difluorophenylamine

(F2C6H3-NH2), we found that graphene can be functionalized during diazotization of amines simply by dipping the graphene sample into the reaction mixture. The functionalized EG on SiC are characterized using x-ray photoelectron spectroscopy (XPS), Raman and scanning tunnelling microscopy and spectroscopy (STM/STS). The combined results suggest that basal

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Figure 1. (a) Schematic of spontaneous reduction of diazonium salt leading to the formation of R/Ar radical, which ultimately chemisorbs onto the material surface. (b) Diazotization reaction of primary alkyl (R) or aryl (Ar) amines. plane of EG on SiC is functionalized with thioethyl (HS-C2H4-) and 3,5-difluorophneyl (F2C6H3-) group when graphene sample was preloaded into the diazotization reaction of

HS-C2H4-NH2 and F2C6H3-NH2 performed in a glass tube, respectively. The present study opens a new chemical route for functionalization of graphene with varieties of active functional end groups, which can be utilized for further capturing and/or immobilization of metal atoms or bioactive molecules onto functionalized graphene. EXPERIMENTAL METHODS EG on SiC was prepared by direct heating of a SiC sample (~ 8 mm x 20 mm) cut from a 6H-SiC(0001) wafer at 1350 oC for 5-7 cycles of 1 min in UHV (maximum pressure ~ 5.0 x 10-9 Torr) chamber. The clean EG prepared in UHV is characterized ex-situ by STM/STS and Raman spectroscopy as reported earlier.21,22 Surface modification of EG on SiC is done in borosilicate glass test tube to avoid any contact of the graphene surface with the wall of the reaction vessel. In a typical set up, a 15 ml test tube is loaded with 5 mmol of desired amine in 5 ml water followed by addition of 10 mmol of conc. HCl, and shaked well for completely 5 ACS Paragon Plus Environment


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dissolving the amine. The aqueous solutions of those amines are colourless. Then the EG on SiC sample is immerged vertically into the tube containing the clear amine solution. The tube is then cooled to below 5 oC in ice bath and slowly added the 5 mmol of solid NaNO2. The solution immediately turned into orange red (for HS-C2H4-NH2) and light blue (for

F2C6H3-NH2) indicating the formation of diazonium salt. The in-situ reduction of freshly formed diazonium salt in solution is visualized by the bubble formation onto the EG on SiC substrate, which gradually lifts the SiC sample to float and re-sink after breaking the bubbles. After ~30 min reaction of the diazonium salt with the EG on SiC below 5 oC, almost 90% of diazonium salt solution is decanted from the tube and immediately added ultra-pure water to the tube containing SiC sample. After few cycles of decanting and dilution processes, the EG on SiC is taken out of the tube and immerged into pure water in a beaker, and gently rinsed the sample. Thus a microscopically clean EG on SiC is obtained. The clean and chemically modified EG on SiC is characterized by x-ray photoelectron spectroscopy (XPS), Raman and scanning tunneling microscopy and spectroscopy (STM/STS). XPS data were obtained with an AXIS-NOVA XPS system using an AlK x-ray source. For XPS measurements, the incident and emission angles were 60 and 0 degree to the surface normal. The analyzer pass energies for wide range and high-resolution measurements were set at 160 and 20 meV, respectively. The analyzer slit was set to 110 m. Raman measurements were performed by Nicolet Almega XR Raman with a 532 nm laser. Raman spectra were acquired with 100X objective, which results in spatial resolution down to 1 m. Ambient STM measurements were performed using a Nanosurf easyScan 2 STM system. The Pt-Ir tip is used for scanning. The STS I-V curves were acquired by sweeping the bias from positive to negative value while the feed-back loop is off. The tip-surface distance is defined by the set-point of the scanning condition. The clean and modified EG on SiC were mounted on the magnetic steel disc using silver paste, which was then attached to the sample holder for STM measurement. In Nanosurf easyScan 2 STM system, bias was applied to the tip during constant current STM measurement.


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RESULTS AND DISCUSSION It is well known that diazonium salts undergo spontaneous reduction on graphene surface.12,15 In present study, the in-situ prepared diazonium salts HS-C2H4NN+Cl- and F2C6H3NN+Cl- are expected to undergo reduction on the preloaded EG on SiC resulting into HS-C2H4- and F2 C6H3- functionalized EG on SiC as shown in figure 2. The –SH functionalized graphene has high technological significance as it can be utilize for further attachment of metal cluster or bioactive molecules leading to the development of graphene based sensors. For chemical characterization of functionalized EG on SiC, detailed XPS investigation was done before and after the functionalization procedure. Figure 3a shows the wide range XPS spectra of (i) the clean EG on SiC and following the treatment with in-situ prepared diazonium salts, (ii) HS-C2H4NN+Cl- and (iii) F2 C6H3NN+Cl- in aqueous medium. For the clean EG on SiC [figure

Figure 2. Schematic of reaction mechanism for spontaneous reduction of thioethyldiazonium (HS-C2H4NN+) and 3,5-difluoroaryldiazonium (F2C6H3NN+) ions on EG on SiC and their respective chemisorbed states.



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3a(i)], only the C 1s, Si 2p and Si 2s peaks resulting from the epitaxial graphene and SiC substrate are observed.21-23 The associated small peaks at higher binding energy side are due to the respective plasmon losses. Absence of any other peaks such as O 1s indicates that EG on SiC is free from any contamination. In the XPS spectrum of HS-C2H4NN+Cl- treated EG on SiC [figure 3a(ii)], the S 2p and S 2s peaks at ~164 and 227.5 eV are observed in addition to C 1s and Si 2p peaks.24,25 High resolution XPS spectra of S 2p region reveals spin-orbit coupling for S 2p peak and single component for S 2s peak as shown in figures 3b and 3c. The S 2p components at 163.5 and 164.7 eV and the S 2s single peak at 227.5 eV suggest the presence of –SH group bonded to C atom as expected in figure 2. 24,25 Small O 1s peak (~530 eV), as seen in figure 3a(ii), always

Figure 3. (a) Wide range XPS spectra of (i) clean EG on SiC, and EG on SiC after treatment with in-situ prepared diazonium salts of (ii) 2-aminoethanethiol (HS-C2H4-NH2) and (iii) 3,5-difluoroaniline (F2C6H3-NH2). The zoomed-in F 1s region is shown in the inset. (b) High resolution XPS spectrum of S 2p region. Computational Voigt fitting to the experimental results are also shown. (c) High resolution XPS spectrum of S 2s region.


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appears when EG on SiC is functionalized in liquid phase and exposed to air before the XPS measurement.21,22 We ascribe this small O 1s peak to the air oxidation of graphene C atoms next to bonding site of HS-C2H4 group. In the case of F2C6H3NN+Cl- treated surface [figure 3a(iii)], the F 1s peak at 687 eV is observed in addition to the C and Si related peaks, which indicates the presence of F containing functional group on graphene surface.26 It should be noted that no Na and Cl related peaks are observed which were present in the reaction mixture as Na+ and Cl- ions, i.e., no chemisorption of Na and Cl onto the graphene surface. Further confirmation of the chemisorbed species on the modified EG on SiC comes from the high resolution C 1s spectra as shown in figure 4. The C 1s XPS spectrum of clean EG on SiC exhibits two major components at 283.7 and 284.5 eV (figure 4i). The lower energy peak (283.7 eV) ascribed to the substrate sp3 C atom bonded to the Si atom.21,23 The 284.5 eV peak

Figure 4. High-resolution C 1s spectra of (i) clean EG on SiC, and the clean EG on SiC in-situ treated with (i) 2-aminoethanethiol and (iii) 3,5-difluoroaniline derived diazonium salts below 5 oC. 9 ACS Paragon Plus Environment


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is due to the sp2 C atom of epitaxial graphene.23 The large background peak tailing at higher binding energy side arises from the sub-surface buffer layer layer C atoms bonded and non-bonded to the substrate Si atoms.23 For the HS-C2H4NN+Cl- treated surface (figure 4ii), a shoulder at ~ 285 eV is clearly realized, which is ascribed to the C-S species of the chemisorbed HS-C2H4 radical onto the EG on SiC (as in figure 2).27 Similarly, for F2C6H3 chemisorbed surface (figure 4iii), the higher binding energy peak at 287 eV is ascribed to the C atom next to C-F unit, which are 3 atoms per chemisorbed species.26 Note that C 1s peak for C atom directly bonded to F atom is expected above 288 eV, which is not observed because of low concentration.26 Indeed the overlap of the sub-surface buffer layer and graphene peaks makes it difficult to accurately estimate the composition of the chemisorbed species on EG on SiC. Considering the atomic sensitivity factors for the F 1s and C 1s peaks, the area intensities of clearly observed two peaks F 1s at 687 eV and C 1s at 287 eV is estimated to be 2:3 as expected for chemisorbed 3,5-difluorophenyl group. Covalent binding of HS-C2H4 and F2C6H3 radical onto the basal plane of EG on SiC is further confirmed by Raman investigation of EG on SiC before and after in-situ treatment with HS-C2H4NN+Cl- and F2C6H3NN+Cl-. The typical Raman spectra of clean EG on SiC and after treatment with HS-C2H4NN+Cl- are shown in figure 5. The Raman spectrum for F2C6H3NN+Cltreated surface is similar to that of HS-C2H4NN+Cl- treated surface (not shown). For clean EG on SiC (figure 5i), two major Raman bands are observed at 1610 and 2740 cm-1, which are ascribed to the G and D band of pristine graphene.28,29 Other peaks observed for clean EG on SiC are originated from the SiC substrate.28 The G band is ascribed to the excitation of in-plane vibration of sp2 bonded C atoms of graphene. The 2D band is the characteristic of defect free clean graphene, which arises from the second-order double resonance process involving two nearest zone-boundaries in the Brillouin zone of graphene.29 In case of any defect creation in graphene structure by chemical binding of molecules, excitation of this band is forbidded, Chemisorption of HS-C2H4 radical onto the graphene surface creats a defect in


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Figure 5. Raman spectra of (i) clean EG on SiC and (ii) the EG on SiC after treatment with in-situ prepared 2-aminoethanetiol derived diazonium salt. The G and 2D bands characteristics of clean graphene, and the defect induced D band are indicated. the lattice, hence, the intensity of 2D band decreases and the defect induced band D appear at 1375 cm-1 as seen in figure 5(ii). Though the overlaps of the G band with the SiC substrate induced band makes the quantitative analysis difficult, the D/2D band ratio (~ 0.8) clearly indicates the significant concentration of defects created by the chemisorptions of HS-C2H4 radical on the basal plane of graphene. Relatively lower intensity of D band induced by the chemisorptions of molecules through reduction of diazonium salt is in agreement with the previous study.20 The surface cleanliness and atomic scale characterization of the modified surface is investigated by ambient STM and STS. Figure 6 shows the STM images of clean EG on SiC (figure 6a), the clean EG on SiC treated with F2C6H3NH2 derived diazonium salt in aqueous solution (figure 6b and 6c). For the clean EG on SiC, epitaxial graphene mesh over the underlying reconstructed 6√3 × 6√3 structure of the SiC substrate is clearly observed. When the EG on SiC is treated with F2C6H3NH2 derived diazonium salt, large number of small 11 ACS Paragon Plus Environment


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Figure 6. STM images of (a) clean EG on SiC and (b) after treatment with F2C6H3NH2 derived diazonium salt. The black square area is shown as (c). (d) STS I-V curves randomly measured on different points of modified surface. Vtip = 0.5 V, Itunnel = 1 nA. protrusions randomly distributed all over the scanning area are observed in the STM image. These small protrusions, as indicated in figure 6c, are ascribed to the chemisorbed F2C6H3 species on EG on SiC. Careful inspection of the images reveals that graphene mesh is visible in the STM image of modified graphene (figures 6b and 6c), which indicates very low concentration of the chemisorbed species. By counting the number of small protrusions in the clearly observed area, we estimate the approximate concentration of the chemisorbed molecules ~4 % atoms. This estimated concentration is in agreement with the XPS and Raman data, which also indicates the small coverage of the chemisorbed species. To see the local electronic states of the modified graphene, the STS I-V measurements were done on different location of surface. Typical STS I-V curves of clean and modified EG on SiC is shown in figure 6d. The measured STS I-V curves are very reproducible as indicated by the similar 12 ACS Paragon Plus Environment

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curves obtained by different measurements. Unlike oxidized EG on SiC, no local band gap are observed but a change in spectral shape is observed after the modification of EG on SiC.30 Hence, the modified graphene with low concentration of chemisorbed species is expected to show the excellent electronic properties as pristine graphene. CONCLUSIONS One of the prominent ways to functionalize graphene and other carbon materials is the reduction of diazonium salts. Normally the diazonium salts are synthesized through diazotization of primary amine at low temperature (below 5 oC). Though there are large numbers of different amines commercially available, as-prepared diazonium salts are limited because most of the primary amines do not give stable diazonium salts. In this study, we focus on in-situ reduction of different types of amine derived diazonium salts on epitaxial graphene on

SiC.

Using

two

sample

molecules,

2-aminoethanethiol

(HS-C2H4-NH2)

and

3,5-difluorophenylamine (F2C6H3NH2), we found that graphene can be functionalized in-situ through diazotization of amines instead of using pre-synthesized diazonium salts. The combined XPS, Raman and STM/STS results suggest that basal plane of EG on SiC is functionalized with thioethyl (HS-C2H4-) and 3,5-difluorophneyl (F2C6H3) groups during the diazotization procedure of the respective amines. The present study opens a new chemical route for the functionalization of graphene with varieties of active functional groups, which can be utilized for capturing and/or immobilization of metal atoms or bioactive molecules.

ACKNOWLEDGEMENT The work is supported by Grant-in-aid from Japan Society for promotion of science (JSPS). (KAKENHI Grant no.16K04883).

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30. Hossain, M. Z.; Razak, B. A.; Yoshimoto,S.; Mukai, K.; Koitaya, T.; Yoshinobu, J.; Sone, H.; Hosaka, S.; Hersam, M. C. Aqueous-Phase Oxidation of Epitaxial Graphene on the Silicon Face of SiC(0001). J. Phys. Chem. C 2014, 118, 1014-1020.



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In Situ Functionalization of Graphene with Reactive End Group

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