Spontaneous Modification of Free-Floating Few-Layer Graphene by

Subscriber access provided by Mount Allison University | Libraries and Archives

Article

The Spontaneous Modification of Free-Floating FewLayer Graphene by Aryldiazonium Ions: Electrochemistry, AFM and Infrared Spectroscopy from Grafted Films Anna K Farquhar, Haidee M Dykstra, Mark Richard Waterland, Alison J. Downard, and Paula Ann Brooksby J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11279 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 13

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

The Spontaneous Modification of Free-Floating Few-Layer Graphene by Aryldiazonium Ions: Electrochemistry, AFM and Infrared Spectroscopy from Grafted Films Anna K. Farquhar,1 Haidee M. Dykstra,2 Mark R. Waterland,2 Alison J. Downard,1 Paula A. Brooksby1* 1. MacDiarmid Institute for Advanced Materials and Nanotechnology. Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. 2. MacDiarmid Institute for Advanced Materials and Nanotechnology. Chemistry – Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand. ABSTRACT: Free-floating, and copper-supported, few-layer graphene sheets were spontaneously modified from an aqueous solution containing nitrobenzenediazonium ions. The infrared spectra of the chemically modified (copper etched) free-floating graphene were measured in transmission mode by manipulating the sheets onto a KBr disc. The major advantage to this method is the ability to release the graphene sheets off the disc to re-float on a water bath allowing the graphene to be further modified or deposited onto a new substrate suitable for other analysis. In this study graphene sheets were then mounted onto HOPG for AFM imaging and electrochemical measurements. The results show there are at least two reaction pathways for spontaneous film grafting to graphene: the commonly accepted aryl radical leading to films containing –C-C– linkages, and a direct reaction of the diazonium cation with graphene to give films containing –N=N– linkages. The ability to manipulate modified graphene sheets onto electrodes with two orientations, with the film exposed to electrolyte solution or sandwiched between graphene and HOPG, leads to different estimates of the surface concentration of electroactive groups. When the film is sandwiched between graphene and HOPG, two electroreduction signals for the nitro group are seen and much larger surface concentrations are measured. This is the first account of such a signal and is tentatively attributed to different peak potentials for reduction of nitro groups at graphene and HOPG. The solution permeability through the graphene sheet and attached films has important electrochemical consequences for systems of this type employed in supercapacitor applications.

Keywords: CVD, surface modification, FTIR, ATR, transmission, KBr, cyclic voltammetry. INTRODUCTION The processability of pristine graphene and few-layer graphene (FLG) is poor because this material is highly hydrophobic and difficult to suspend in most common solvents. Graphene and FLG also aggregate in solution which further complicates processing. Thus, synthetic methods that allow the controlled chemical modification of these materials to improve handling, and at the same time introduce desirable functionality for target applications, are advantageous.1 Modifying carbon surfaces using aryldiazonium ion chemistry is a well-established technique.2-4 The reaction proceeds by diazonium ion reduction to a reactive radical

species that attacks a surface near where it was generated. This radical can be initiated chemically and electrochemically, but also spontaneously at conductive surfaces. Reductive grafting gives unpredictable film growth because the radical can also react with neighboring solution species to produce polymeric physisorbed matter, or it can attack the just formed films to generate multi-layered structures.5 More recently two spontaneous cation-based covalent modification pathways are being investigated; one route is the heterolytic dediazoniation to give a reactive aryl cation (for non-aqueous conditions), and the second is direct reaction of the diazonium group with an electron rich substrate or film.6 The second cation-based pathway would explain why azo linkages can be found during x-ray

ACS Paragon Plus Environment


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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

photoelectron spectroscopy (XPS) studies in diazonium generated films on many substrates. Modifying pristine graphene via aryldiazonium chemistry is usually performed after graphene has been anchored onto a substrate that allows for easier handling and processing.7-13 However, substrates are known to introduce local regions of varying electron density across the graphene surface, which subsequently moderates the grafting behavior and films of variable density can be generated.14 Aryldiazonium grafting is not edge specific at graphene, though it is known that edges are more reactive than the basal plane, and that single layer graphene is more reactive than bi-layer graphene.12, 15 Thus, FLG could be anticipated to behave more like highly ordered pyrolytic graphite (HOPG) surfaces when it reacts spontaneously with aryldiazonium ions. The spontaneous grafting of aryldiazonium salts to substrate supported (single-, bi-layer and epitiaxially grown few-layer) graphene was observed by Raman, IR and XPS where it was evident the aryl rings reside on the basal plane and that this route to graphene modification appears to be effective. 7, 16-17 Our interest is with FLG as a material for energy storage applications rather than single layer graphene. FLG and defective graphene materials were calculated to have improved capacity for supercapacitor applications.18 Architecturally designed interfaces using FLG will require chemically assisted construction and assembly, and diazonium-based grafting routes offer a simple and effective approach for this application. In this article we describe a new approach to handling spontaneously grafted films on FLG sheets using a simple KBr disc. The infrared spectra from modified FLG sheets can be easily, and repeatedly, measured. The graphene sheets can then be removed from the KBr disc and reused for other measurements, in this instance AFM and electrochemical measurements. Additionally, free-floating FLG sheets are used in this work because the grafting process can be substrate-less so that additional chemical and mechanical sheet manipulations can be developed. EXPERIMENTAL METHODS Chemicals Ultra high purity (UHP) methane, hydrogen, and argon gases (BOC, New Zealand). Copper foil (25 µm thick, 99.999%, Alfa Aesar), ammonium persulfate (Merck), p-nitroaniline, disperse orange 3, tetrafluoroboric acid, sodium nitrite, sulfuric acid (J.T. Baker), methanol and isopropanol (Merck) were used as received. Diethyl ether (Sigma Aldrich) was dried prior to use. Electrolyte solutions were prepared using MilliQ water, > 18 MΩ. cm. Potassium bromide (KBr) discs (Sigma-Aldrich) were cleaned with methanol on a polishing pad prior to use. HOPG surfaces (SPI supplies) were freshly cleaved prior to use.

Page 2 of 13

CVD growth of graphene and etching from copper The procedure for the atmospheric pressure (AP) CVD growth of FLG was derived from Wu et al.19 Prior to growth, copper foils (25 µm thick, 99.999%, Alfa Aesar) were cleaned in dilute nitric acid solution, then MilliQ water (> 18 MΩ.cm) and finally isopropanol (Merck), and then dried with nitrogen. The foil was cut into approximately 1 cm × 1 cm pieces and placed in a furnace within a silicon glass tube. The furnace was first heated to 100 oC for 30 min, under 400 sccm Ar (UHP, BOC, New Zealand) and 50 sccm H2 (BOC, New Zealand). The furnace was then ramped to 1050 oC under 500 sccm H2, and held at this temperature for 30 min. Methane (UHP, BOC, New Zealand) at 5 sccm was introduced for 3 or 7 minutes depending on the number of layers required. After this growth phase, the furnace was cooled for 1 h to 600 oC under 500 sccm H2. The H2 was switched off and the samples were pulled out of the heating zone of the furnace, but still within the furnace tubing, to cool rapidly to room temperature. The copper was etched using 0.1 M ammonium persulfate (Merck) solution for 10 min to release the FLG from one side. The foil was then floated on a fresh 0.1 M ammonium persulfate solution overnight, leaving a single sheet of FLG. This was then collected in a watch glass and rinsed five times with water by pipetting out the old solution and replacing with water. The 7 min growth protocol gave 6 to 7 layered graphene sheets. The general sequence of steps from etching, to modification and infrared, AFM and electrochemical analysis of FLG samples are shown in Scheme 1. For Raman, UV-visible and XPS analysis the HOPG substrate was replaced (step 6) with either gold or silica. Graphene modification Once rinsed, the FLG, still in the watch glass, was floated on an aqueous aryldiazonium salt solution and left in the dark for known time periods. After modification the FLGmod was then rinsed five times with water using a pipette to remove and replace the water solution, and then transferred to a large dish containing water where it remained for 20 min. The FLGmod sheet was transferred to a desired substrate by lifting the substrate from below the sheet floating in the water bath. After drying, a quick and gentle methanol rinse step was performed and the substrate re-dried. The substrates used in this study were KBr (for IR spectroscopy), HOPG (AFM and electrochemistry), quartz slide (UV spectroscopy), Si/SiO2 (XPS), and Au (Raman spectroscopy). KBr disc Cu/FLG Etch bath

(1)

FLG Modification solution

(2)

Water

(3) HOPG (6)

Water

(5)

-

Diazonium salt preparation The aryldiazonium (BF4 ) salts were prepared by a standard procedure.2 A description is given in the Supporting Information.

(4) Infrared

(7) CE

Epoxy seal (9) (8) AFM


RE

2

(10) WE (11) Electrochemistry

Page 3 of 13

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SCHEME 1. The sequence of steps that each Cu/FLG sample undergoes beginning with etching the copper then (1) rinsing with water and then replacing with the aryldiazonium modification solution, (2) collecting onto a KBr disc, (3) drying and (4) collecting the IR spectrum, (5) re-floating in a water bath, (6) collecting onto an HOPG substrate, (7) drying and imaging with the AFM, (9) sealing the graphene edges with epoxy resin, (10) mounting into an electrochemical cell, (11) measuring the electrochemistry. See text for Raman, XPS and UV-visible protocols.

For FLG modified while on copper, the copper coupon with one side containing FLG was floated on a solution of the aryldiazonium modifier for a known time and then removed and rinsed (aqueous and methanol) and finally etched and rinsed as described above. Modifying graphene while it is still on Cu allows control over the film orientation with respect to a substrate, and modifier groups can be mounted in an exposed configuration, as compared to the free-floating FLGmod method which gives films sandwiched between the graphene sheets and a substrate, Scheme 2. Etching a copper-FLG sheet with the FLG face-down in the etchant bath tends to break the sheet structure apart so was not used. AFM AFM (Digital Instruments Dimensions 3100) topographical measurements were done in non-contact tapping mode with a silicon cantilever (Tap300Al-G) operating at resonant frequencies between ~280 kHz. Images were collected with a high resolution (512 samples per line) at a scan rate of 0.5 Hz.

Free floating modification

On copper modification

FLG

Cu/FLG

Modifiers

Grafted film Water bath

Modifiers

Orientation of FLG & film in etchant bath

Wash after modification Substrate

Mounting onto substrate

“sandwiched" Final orientation

“exposed”

SCHEME 2. Orienting free-floating and copper-backed

FLG sheets to give control over the final interface structure with modifier groups being sandwiched or exposed.

Electrochemistry Electrochemical measurements were made using an Autolab potentiostat running NOVA software. The electrochemical cell was set up with the substrate (HOPG/FLG) placed horizontally between an insulated metal base plate and a glass solution cell. A copper foil electrical contact was connected to the substrate. An o-ring and four springs from the plate to the cell sealed the electrolyte above the sample. For all electrochemical measurements, a large area gold wire electrode was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All solutions were sparged with N2 for 15 minutes prior to measurement at 20 ± 2 °C. IR spectroscopy All IR measurements were performed using a Bruker Vertex 70 spectrometer operating OPUS software. Spectra were recorded as absorbance in either transmission or ATR mode. Spectra were collected using 32 scans at 4cm-1 resolution from 600 to 4000 cm-1 using a liquid N2 cooled MCT detector. Only the regions from 1700 to 1100 cm-1 and 4000 to 2500 cm-1 are shown as these contain the most prominent absorptions. A nontransparent cutout was used to define the cross-sectional area of the IR beam passing through the sample (0.8 cm2). The peak-to-peak absorbance for a 100% line at 1344 ± 50 cm-1 in transmission mode with a KBr disc is 0.0002. This wavenumber corresponds to a vibrational mode for the NO2 group that is mostly isolated from other peaks in the spectrum. Hence, a peak greater than ~0.0006 absorbance at this wavenumber is considered discernible and used to determine the IR detection limit for this technique under these conditions. A peak absorbance of 0.0006 (assuming a linear molar absorptivity correlation for all film thickness) equated to a minimum NO2 surface concentration ~1 × 10-10 mol cm-2 based on correlated electrochemical data. This calculation also presumes the nitrophenyl (NP) film is randomly oriented on G which may not be true for a very thin (monolayer) of aryl-NO2 groups as compared to thicker multilayered and physisorbed films. UV spectroscopy The UV spectra were recorded on a Cary 50 UV-Vis Spectrometer. Data were collected from 800 to 400 nm at a rate of 600 nm.min-1 at 1 nm resolution. Raman spectroscopy Raman spectra were measured using a 532 nm (2.33 eV) laser with 5 mW power in a confocal microscope arrangement with a 40× objective lens and a numerical aperture of 0.65. The spot size was ~2 µm. Spectra were collected in the backscattering mode using 20 exposures at 5 s exposure times. Preparation of samples for microscopy, spectroscopy, and electrochemistry For IR spectroscopy, the FLG sample was collected from the water bath onto a freshly prepared KBr disc and dried at

3



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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60 oC for a minimum of 15 min, then rinsed with methanol and dried for a further 10 min. After IR measurements, the KBr discs were re-submerged in a water bath to refloat the graphene, which could then be recollected onto different substrates for other analyses, Scheme 1. For spectroscopy and microscopy the FLG sheets were mounted onto their appropriate substrates and dried as described above. For electrochemistry, the FLG was mounted onto HOPG, and a two-part epoxy resin (EPO-TEK 301) was used to seal a circular working electrode area (0.85-1.5 cm2). The resin was dried for 2 h at 60 oC followed by 22 h at room temperature and the area exposed determined by collecting an photographic image for software analysis (ImageJ). Peak fitting Voltammetric peaks from surface species were curve fit using the Levenberg-Marquardt algorithm with mixed Lorenzian-Gaussian peak shapes. A non-linear polynomial baseline was fitted and subtracted prior to the curve fit. Three approaches to the curve fit were examined: (i) treating the baseline as a linear function of the pre-peak voltammetric slope then adding peak(s) to simulate the voltammogram; (ii) simply using the preand post-peak current-potential trace as the baseline and fitting a peak to this baseline; (iii) using the second voltammetry cycle as the base to subtract from the first and computing the area between the curves for a defined switching potential. Method (i) was reasonable only for large voltammetric peaks, but smaller peaks were significantly overestimated. Likewise, method (iii) was rejected due to unknown capacitance contributions to this baseline and baselines similar to (i) that became unrealistic for small voltammetry peaks. Method (ii) was chosen for all calculated data.

Page 4 of 13

type effects. The undulations across the rings are ~ 5 nm high. iii) Circular objects that are commonly up to 20 nm but a few that are much taller. The origin of these features is likely carbon-based structures such as balls or amorphous particles and folds and wrinkles from the transfer process. AFM line profiles across these images in Figure 1 are given in Figures S3 to S6 of the Supporting Information. The number of graphene layers was determined by the reduction in UV transparency according to the method of Geim et al.21 and the average number of layers was 6.5 (Figure S7 in Supporting Information). The Raman spectra from FLG sheets mounted onto gold substrates are given in the Supporting Information, Figures S8 and S9. The spectra are consistent with FLG containing some defects and comparable to spectra from graphene made by similar AP CVD methods.

A

B

C

D

RESULTS AND DISCUSSION Characterizing FLG sheets by AFM, and UV and Raman Spectroscopy The atmospheric pressure CVD growth method generates FLG sheets with no measurable graphene oxidation (IR results vide infra). Each sheet contains polycrystalline graphene, which is a collection of smaller single crystal graphene domains held together by disordered grain boundaries with irregular shapes.20 The graphene domains are moderately cohesive and the entire FLG sheet remains intact throughout the rinsing, transfer and analyses steps. AFM images of FLG as grown on copper coupons are shown in the Supporting information, Figure S1. Some graphene crystals with approx. 2-6 µm dimensions are clearly observed. The appearance of the FLG sheets after aqueous transfer onto HOPG is given in Figure 1A. HOPG itself is featureless (Figure S2 in Supporting Information) aside from sharp steps between the basal planes. Three types of topography are evident in Figure 1A: i) Creases and folds which appear as white lines in the image. The tallest few are ~ 20 nm high, but most are lower than 6 nm. ii) Large hexagonal shapes with ring

FIGURE 1. AFM images of (A) FLG on HOPG; (B) FLGNP on HOPG with the NP groups between FLG and the HOPG; (C) FLGNP on HOPG with the NP groups facing outwards (exposed), and (D) An exposed FLGNP surface at higher magnification. FLGNP modifications were in 20 mM NBD for 20 h. Methanol rinsed surfaces. Scale bars are (A-C) 2.5 µm, (D) 110 nm.

Modifying FLG sheets using Aryldiazonium Ions. We modified free standing graphene by floating the FLG sheet on a solution containing the aryldiazonium ions for known times from 1 h to several days in concentrated (20 mM) solutions. In a second approach, we modified FLG while it was still on the copper substrate in the aryldiazonium solution. Copper etching was performed after the aryl film was grafted, Scheme 2. The methanol rinse step removes a large portion of physically adsorbed material, based on the average peak area of the NO2 electrochemical reduction signal between replicate

4


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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


samples that were rinsed and non-rinsed, but also based on infrared data comparing rinsed and non-rinsed samples. Simple rinsing in water or water-isopropanol mixtures did not remove this extraneous material. Spontaneous grafting of nitrobenzenediazonium BF4 salt (NBD) to free-floating FLG results in a NP modified FLG sheet (FLGNP) mounted onto an electrode with the NP groups sandwiched between the electrode and FLG substrate, Scheme 2. Figure 1B is an image of FLGNP mounted in this orientation and there is little difference between the AFM images of FLG and FLGNP. By contrast, FLGNP formed with FLG still on the copper and then etched and mounted such that the NP groups are outermost clearly show the presence of NP groups, Figure 1C, with the topography suggesting a significant NP film coverage. Figure 1D is a higher resolution scan of a film which appears to have semi-regular periodicity that is assumed to be molecules, or small groups of molecules, anchored to graphene. An aryl-NO2 modifier group is calculated to be ~0.7 nm high if vertical. The spherical features in the AFM image are between 20 and 25 nm apart and ~ 1 nm high; though how high from the graphene substrate is not determinable. Their regular width of 10-13 nm is mostly likely reflecting the curvature of the AFM tip. AFM imaging of these aryl films can be deceptive because no direct information about the nature of the bonding to graphene can be inferred even though it is clearly evident that a film resides on the surface. Grain boundaries between the graphene crystals are defect areas that provide sites for aryl modification. At HOPG, the grafting of aryl species to the surface is largely at these defects and at edge sites and until recently there was little evidence for significant basal plane modification.5 However, De Feyter et al. have shown basal plane modification does occur at HOPG by observing the sub-layer lattice distortions using STM imaging, and showing that these distortions disappear when the aryl group is removed from the surface using the STM tip.22 Raman studies with graphene suggest aryl grafting to the basal plane must occur,23 and Stark et al. show aryl grafting to single layer graphene basal planes is faster than at bi-layer graphene, and that physically adsorbed products also reside at the surface.12 Hence, the reactivity and spatial density of grafting aryl groups onto single-, bi-, and few-layer graphene and HOPG substrates depends on the substrate type and defect populations, but none-the-less give quite similar films when ≥ 2 defective graphene sheets are present. Monitoring the grafting process and the subsequent effects on graphene structure is most commonly performed by Raman spectroscopy. The Raman spectrum from a single layer of pristine graphene has two bands, one at 1580 cm-1 (G peak) and the other at 2670 cm-1 (G’ or 2D peak), and after a chemical reaction a prominent band at 1336 cm-1 (D peak) emerges.17 The D-band is due to sp2

bonding in the six-carbon rings that becomes Raman active after neighboring carbon atoms become sp3 hybridized. Thus, the D-band is indicative of bonding to the basal plane of graphene, or defective graphene planes and edges. Thus, the D/G ratios are used as a measure of the inherent defects, or a change to the number of defects, following chemical modification. The G’ band shape can also be indicative of the number of graphene layers.24 The Raman spectra of FLG, and FLGNP samples modified for 2 and 72 h are shown in the Supporting Information, Figures S8 and S9. There is little difference between the 2 and 72 h spectra suggesting FLG surfaces are no longer being significantly modified beyond 2 h (though the infrared and XPS results, vida infra, indicate film growth on top of the initial layer continues). A plot of the D/G ratio from spectra collected every 500 nm across 10 µm FLG and FLGNP surfaces is shown in Figure 2. The average D/G values for FLG and FLGNP are 0.13 ± 0.01 and 0.25 ± 0.03, respectively. The increased D/G ratio following modification is expected and shows the basal plane was chemically modified. Spectra were also collected from FLG (N=4) and FLGNP (N=3) samples, and from 3 locations on each sample. The between sample and location variations were small compared to the D/G values measured between FLG (0.14 ± 0.03) and FLGNP (0.26 ± 0.04).

0.3

D/G ratio

Page 5 of 13

FLGNP 0.2

0.1

FLG

0.0 0

1

2

3

4

5

6

7

8

9 10

Distance / µm FIGURE 2. The Raman D/G ratio from FLG and 72 h modified FLGNP surfaces taken every 500 nm across a 10 µm section of the surface.

The nature of the film on the surfaces and the type of bonding to the FLG sheets was next examined using IR and XP spectroscopy. The XPS C1s and N1s core level spectra of FLG, FLGNP (modified for 2 and 72 h), and a 72 h iodophenyl modified FLG (FLGIP) surface are shown in Figure 3. The inclusion of the iodophenyl surface was to observe the N1s region more clearly without a complication from the NO2 signal. Recent work has shown the diazonium grafting mechanism is not limited to the classically accepted reduction reaction to the aryl

5


The Journal of Physical Chemistry radical species, but also includes the direct reaction of the diazonium cation onto suitable surfaces.6 Indeed in the FLGIP surfaces we observe a N1s peak typical of a –N=N– group at 400 eV, Figure 3. The O1s spectra (not shown) have peaks due to the silica substrate (533 eV) and (for FLGNP) NO2 (536 eV). A table of peak analysis data is given in the Supporting Information, Table S1. The carbon spectra show a new signal appearing at higher binding energy after modification with nitro- and iodophenyl groups. The new peak at 285.5 eV is consistent with aryl carbon signals of the type C-O and C-N. This peak is relatively larger for the FLGNP modified for 72 h compared to that at 2 h, and comparable to the 72 h iodophenyl modified surface. The N1s spectra have two distinct peaks; one at 400 eV that we are assigning to the –N=N– group and appears in all modified spectra, and one at 406 eV assigned to the NO2 group which is absent in the FLG and FLGIP spectra. The relative area of the – N=N– and NO2 peaks in both FLGNP modified surfaces is close to 1 to 1 suggesting that for every azo group in the film, there are two nitro groups present. Hence, both radical aryl and the direct diazonium-based grafting routes are operative. A similar ratio has been found for spontaneously grafted NP films at other carbon materials.25

2.5 2.0

C1s

1.5

FLGIP, 72h FLGNP, 72h

1.0

FLGNP, 2h

0.5

Modifying free-floating FLG presents some challenges for subsequently measuring the IR spectra from those surfaces. Collecting the modified sheets onto a reflective substrate such as gold to measure the spectra in the reflection-absorption or attenuated total reflection (ATR) modes is an option but that leaves the film unusable for subsequent procedures. Here we developed a simple approach where the FLG was collected onto a KBr disc and the transmission spectrum recorded. There are four important advantages to this method: transmission mode samples the film in its entirety and given knowledge of the absorptivity one could determine the number of groups giving rise to a particular vibration directly; the FLG sheets can be released from the KBr substrate by simple immersion into water, and thus the same FLG sample can be used repeatedly for chemical synthesis or other analyses (Scheme 1); furthermore, transmission spectra can be collected with all general IR instruments whereas, reflection-absorption and ATR requires specific accessories; and lastly, the ATR reflection element is commonly diamond and in those cases an important region of the mid-IR spectrum of multiply bonded nitrogen species (from ~1900-2300 cm-1) is unable to be measured due to absorption by the diamond bands. More details regarding the transmission method are given in the Supporting Information, including demonstrating how the KBr substrate can affect the baselines, and that spectra obtained using ATR and transmission mode are comparable. 1540 1356 1318

FLG

0.01

0.0 283 284 285 286 287 288 289 1.0 0.8 0.6 0.4 0.2 0.0

1615

Absorbance

Normalized CPS (a.u.)

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 13

N1s

D C 1344 1518

1327 1168 1111

1508 1406

396 398 400 402 404 406 408

B

1610

1243

Binding Energy / eV FIGURE 3. XPS spectra from FLG and spontaneously grafted FLGNP (2 and 72 h) and FLGIP (72 h). Methanol rinsed.

The IR spectra of modified FLG surfaces provides complementary information to the Raman. Combined with XPS results, the IR can be a powerful tool for film analysis.

A 1700 1600 1500 1400 1300 1200 1100

Wavenumber / cm-1 FIGURE 4. The IR spectra of (A) FLG, (B) FLGNP, 17 h modification and no methanol rinse, (C) FLGNP, 24 h

6


Page 7 of 13

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modification with a methanol rinse. (D) BF4.N2-C6H4NO2 salt collected in ATR mode and scaled by 0.08. Grafting solutions used aqueous 20 mM NBD. Spectra are offset for clarity.

Figure 4 are the transmission IR spectra for (A) FLG, (B) FLGNP without methanol rinsing (water rinse only), (C) FLGNP after methanol rinsing, and (D) an ATR spectrum of NBD. The spectrum of CVD FLG on the copper substrate is largely featureless in the wavelength regions of interest, but importantly, no bands are present that would indicate graphene oxide (Supporting Information Figure S13A). Upon transfer of etched FLG from solution to the KBr substrate, an intense bipolar band centered at 1585 cm-1 appears due to a Fano resonance, Figure 4A.26-27 Fano resonances arise in layered 2D systems from the inplane optical phonon interactions between discrete and continuum states. Why these resonances appear on KBr but not on copper is unknown at this time. The aryldiazonium method of chemically modifying freefloating FLG clearly gives a significant amount of film at the interface based on the IR spectrum of water rinsed FLGNP shown in Figure 4B. This spectrum has peaks that are far more intense than the methanol rinsed sample in Figure 4C indicating that methanol is a suitable solvent to remove weak and physically bound byproducts of film formation, and after methanol rinsing only strong intensity peaks originating from the grafted film remain distinguishable. This effect is also seen with films grafted to FLG on copper and measured using ATR (Figure S13E). No peak in the N2+ vibrational region (2250 – 2350 cm-1) was observed for modified surfaces indicating unreacted diazonium groups are absent. In general, the methanol rinsed spectra of NP modified FLG substrates are similar to those for NP films formed on other types of carbon substrates and conductive electrodes cleaned using more aggressive methods such as sonication.28-31 Bands arising from byproduct or weakly bound species that are easily removed by rinsing are informative of the types of structures generally formed during the grafting process. Thus their spectra are shown and discussed along with the covalently bound film that remains after rinsing. All band assignments are based on literature accounts and given in Table 1.32, 34, 36-42 A more in-depth discussion of the band assignments is provided in the Supporting Information. The symmetric and asymmetric NO2 vibrations in the NBD spectrum are intense peaks at 1356 and 1540 cm-1, respectively, Figure 4D.32-35 In the films, these vibrations shift to 1344 and 1518 cm-1, respectively. Nitro vibrations are sensitive to aryl substitutions and the 1327 cm-1 peak is speculated as due to nitro groups in a configuration other than a 1,4 ring substitution pattern, or from rings substituted with –N=N– linkages rather than –C-C–. The peaks appearing in the 1500 to 1400 cm-1 region are typical of aryl ring deformation modes. The 1450 cm-1 band present in non-rinsed samples, and absent

thereafter, has been attributed by Pinson et al. to azo linkages from the aryldiazonium grafting process.43 The azo mode vibration is not typically intense in IR spectra due to symmetry considerations and may be below the detection limit in the spectra of the rinsed samples. Of interest in Figure 4B (and 4C though they are far weaker in this spectrum) are the three new intense peaks in the 1300 to 1100 cm-1 region. The 1243 cm-1 peak is assigned to the ν(C-C) mode which is expected to appear when aryl groups graft to FLG as well as to aryl groups in the just formed film. The 1168 cm-1 band is broad and can be fitted with two peaks centered at 1172 and 1161 cm-1. These two bands are assigned to δ(C-H) deformation vibrations.36-39 The third intense peak at 1111 cm-1 is assigned to the C-N vibration of the phenyl-NO2 bond coupled with a C-H wag motion. TABLE 1. Assignments of infrared vibrations appearing in Figure 4 for FLG, FLGNP and NBD species. NBD

FLGNP

1615 m

1610 w

1574 m

Assignment ν (C=C)

1540 vs

(1522) 1518, 1508 s,db

νa (NO2)

1467 w

1484, 1472 m,db

Ring def +C-H wag

1450 m

Ring def +(C-N=N)*

1421, 1409 w

1406 m

Ring def modes

1356 vs

(1347) 1344, 1327 s,db

νs (NO2) νs (NO2) mode

1318 vs

+

ring

1301 m,sh 1295 m 1243 s

ν (C-C)

1187 m,sh

1121 m,sh

1168 s,db

δ (C-H)

1126 w, sh

ν (C-NN)*

1111 s

ν (C-NO2) + C-H wag

* See text for description of assignment. v=very; s=strong; m=medium; w=weak; sh=shoulder; db=doublet. To summarize the spectroscopic data: there is clear evidence for grafting to graphene via –N=N– linkages in addition to the traditional aryl grafted –C-C– route. The former grafting route has not previously been observed at graphene. For every azo linkage in the film there are two NO2 groups present, and they appear to reside in two types of ring arrangements. The origin of IR sharp bands

7


The Journal of Physical Chemistry between 3000-4000 cm-1 is unassigned at this time. Lastly, the formation of weakly bound and physisorbed matter is dominant but simply removed using a brief methanol rinse. Electrochemistry at modified FLG sheets The FLGNP surfaces can be constructed into working electrodes with NP groups either exposed to the 0.1 M H2SO4 electrolyte solution, or sandwiched between the HOPG and FLG sheet itself (Scheme 2). For the case where the redox groups are sandwiched between these two surfaces we assumed the electrolyte would be inaccessible to the NP groups which would therefore be electroinactive. However, the electrochemistry from these groups was clearly observed. This implies the FLGNP, and presumably FLG, sheets were permeable to the solution, most likely via holes and defects. 50

0

j / µA cm-2

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2 -50

-100

1

"exposed" "sandwiched"

-150 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Potential (SCE) / V FIGURE 6. Cyclic voltammograms in 0.1 M H2SO4 of FLGNP mounted onto HOPG with either the NP groups sandwiched between HOPG and the FLG sheets (red; 66 h mod), or exposed to the electrolyte solution (black; 20 h mod). N2 sparged solution. Scan rate = 200 mV s-1. For all films electrochemically analyzed with NP groups exposed to the electrolyte solution, and films prepared using short modification times and then analyzed with NP groups in the sandwiched orientation, the response appeared similar to that shown by the black CV in Figure 6. The voltammetry shown in Figure 6 (black CV) is typical for NP modified carbon surfaces where the irreversible NO2 reduction peak is observed at -0.57 V on the first scan and absent on the second.2 The red CV in Figure 6 is typical of those obtained for all FLGNP samples prepared using long modifying times, and sporadically shorter modifying times, and subsequently analyzed with FLGNP in the sandwiched orientation. The two reduction peaks are both assumed to be due to reduction of NO2. For both CVs, reduction of NO2 is to NH2, however this reaction is incomplete as evidenced by the redox couple

Page 8 of 13

close to 0.25 V ascribed to the NHOH / NO pair, and a second redox couple appearing at more positive potential that is unassigned at this time. The latter couple can be observed in some publications,44-45 though its origin is not explicitly discussed. Compton et al. observed double redox couples of this type at NP modified carbon nanotubes and speculated the more positive couple arises from the NP bound via an ester link when a carboxyl surface group is initially present.46 However, in this instance no COOH groups are on FLG. As seen in Figure 6, this couple was better defined and appeared at a slightly more positive electrode potential when the NP groups were exposed to the electrolyte. Their apparent shift with electrode orientation may indicate the electron transfer kinetic differences between NHOH / NO groups are causing the second couple. This possibility is considered further below with respect to the two NO2 peak reductions. A plot of the NO2 surface concentration, determined using the NP reduction and NHOH oxidation peak areas, against grafting conditions is commonly performed as a measure of defining how much film is present.2 All plots of this type have the general appearance of an initial growth phase that quickly plateaus to some limiting value, regardless of whether the films were generated spontaneously or electrochemically. It is generally accepted that these films continue to grow but cannot be fully interrogated electrochemically because they are not conductive through to the outer film edge, or that NP (or other redox) groups become buried and inaccessible to the electrolyte and hence the plateau effect to these plots. A plot of the NO2 surface concentration determined from voltammetry for FLGNP films mounted conventionally in the exposed orientation are given in Figure 7 (black), and show this usual growth and plateau response. The plateau is at ~5 × 10-10 mol.cm-2 and is in keeping with values determined for films generated under similar solution conditions.2, 35, 47 But, when the NP groups are in the sandwiched orientation, the plot (using both reduction peaks) shows a linear growth relationship with modifying time for up to 72 h, Figure 7 (red). In addition, films mounted with exposed NP groups, and then covered with a conductive FLG sheet (in electrical contact with the HOPG working electrode) prior to analysis, give larger peak areas as for the sandwiched films, Figure 7 (blue data points). However, only a single reduction peak at -0.60 V was observed for these films. Longer modifying times presumably generate thicker or denser films and by having two conducting surfaces either side of the film in the sandwiched FLG electrode orientation, all of the NP groups are electroactive. This implies the conductivity through these films can be limiting, and one could imagine that for a certain film thickness or density, a situation may arise where even a sandwiched interface still underestimates the number of redox groups simply because the center sections of the film are not electrochemically active. Using AFM, the

8


Page 9 of 13

approximate thickness of these films mounted onto HOPG was measured at the edges and found to be between 4 and 6 nm for long modifying times. This is not an especially thick film and implies other factors such as film density may be important.

Surface conc, Γ / 10-9 mol.cm-2

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 "sandwiched"

1.5

1.0

0.5 "exposed" 0.0 0

10

20

30

40

50

60

70

Modifying time / h FIGURE 7. The relationship between FLGNP spontaneous modification times and the surface concentration of NO2 groups determined from films mounted onto the HOPG working electrode in two orientations; exposed (black) and sandwiched (red). Blue data points are from exposed films with a FLG sheet mounted over top. All samples were methanol rinsed prior to mounting in the electrochemical cell. The observation of two nitro reduction peaks for films prepared with long modifying times and then analyzed by sandwiching between FLG and HOPG is very intriguing. The peak potential differences are consistently 200 mV. The same two peaks were observed for films sandwiched between FLG and another graphitic electrode made from pyrolysed photoresist film (PPF). On the other hand, when NP films were electrochemically grafted to glassy carbon (GC) using a long grafting time and subsequently covered with a FLG sheet, only single reduction peaks (at -1.0 V) were obtained. Electrochemically-determined surface concentrations were consistent with the high values obtained in the ‘sandwiched’ configuration (Figure 7). The XPS results imply a mixture of film structures incorporating aryl-aryl and aryl-N=N-aryl linkages. We know the NO2 reduction potential for NAB films is almost identical to those of NP films,45 thus, the two types of aryl substitution patterns is not the origin for the two reduction peaks. Another possibility is that some NP groups are residing in non-polar or hydrophobic environments within the film.45, 48 A study of NO2 reduction potentials from NAB films on carbon electrodes show the NO2 reduction peak can shift by several hundred mV when an overlayer of a low dielectric molecular species is grafted.45 However, the peak shifts vary in magnitude depending exactly on the nature of the

polar environment, but in this study the second peak is always 200 mV shifted from the first. A third possibility arises from the type of aryl substitution pattern; the electrochemical reduction of 2,4,6-trinitrotoluene in aqueous acidic solutions has three reduction peaks, each separated by ~160 mV,49 thus, bi- and tri-aryl substitutions within the film could be the origin of the two peaks, but this does not explain the observation of a single NO2 reduction peak for films represented by the blue data points in Figure 6, that is, films in contact with two FLG sheets. Hence, we tentatively suggest that the origin of the two NO2 reduction peaks (and possibly the two NHOH / NO redox couples) comes from the different electron-transfer (ET) kinetics between FLG and a second electrode material for redox species in a film of sufficient thickness. That is, for thicker films grown on FLG, then placed face-down onto HOPG or PPF, the ET kinetics across the graphite interface to the NO2 groups residing at the outer edges of the film (which are adjacent to the graphite) are slower than ET from the FLG surface to NO2 groups close to (and covalently bound to) the FLG interface. It appears that when the second electrode is GC, the ET kinetics across the two interfaces result in a single reduction peak. When the NP films are sufficiently thin then all of the NO2 groups can be electrochemically reduced via the FLG interface (or NP groups may not make adequate contact with the second electrode) and we do not observe the second reduction peak in those cases. When a relatively thick film is grown on FLG and overlaid with another FLG sheet, Figure 6 (blue data points), both interfaces are the same, with relatively fast ET kinetics, and just the one reduction peak is observed. Although our data are consistent with the interpretation above, it is possible that the extent of intimate contact between the film and the electrode materials plays an important role in the observed response. Physical contact may differ depending on the configuration of the sandwich (film grown on FLG or GC), and the rigidity of the substrate (films sandwiched between two FLG sheets, or between an FLG sheet and PPF or HOPG). CONCLUSIONS This study describes a new approach to handling fewlayer graphene sheets between different solution conditions on a KBr plate that allows IR spectroscopy to be used at each step to interrogate molecular films on their surfaces. In this instance the graphene sheets were chemically modified using aryl diazonium chemistry and after IR spectroscopy they were deposited onto HOPG for additional AFM imaging and electrochemical analysis. We demonstrate the transmission IR spectra from physisorbed and covalently bound films on FLG are clearly measurable and the method provides a simple platform from which to perform repeat measurements with the same sample if desired. XPS and IR spectroscopy

9



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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

show both –C-C– and –N=N– linkages in the film proving at least two grafting mechanisms are operating for spontaneously formed films. We show there is electrochemical discontinuity through these films by measuring different amounts of reducible nitro groups depending on whether there are electrodes contacting the films on one or two sides. We further demonstrate the electroactive nitro group has two reduction potentials that we speculate arise from ET kinetic differences between the HOPG and FLG interfaces. Of significance for electrochemistry, and electrochemical capacitor applications, is the permeability to electrolyte solution through the modified FLG electrode. For the type of CVD grown sheets used in this study, solution permeability for multilayered systems is a distinct advantage for capacitor applications and can be exploited as a means of layering graphene sheets while controlling or eliminating their propensity for coalescence and hence reduced practical electrode area. Furthermore, the aryldiazonium chemistry is a scalable process suitable for large graphene area applications. We are continuing to study the potential of these FLG sheets for this application. SUPPORTING INFORMATION This material is available free of charge via the Internet at http://pubs.acs.org. The material includes AFM images of graphene and film topography, Raman and UV-visible spectra, a table of XPS results, and a detailed description of the transmission IR technique with baseline correction, and comparative ATR and transmission IR data. ABBREVIATIONS AFM, atomic force microscope; ATR, attenuated total reflection; CVD, chemical vapor deposition; FLG, fewlayer graphene; FLGNAB, nitroazobenzene modified graphene; FLGNP, nitrophenyl modified graphene; FLGmod, modified graphene; G, graphene; GO, graphene oxide; HOPG, highly ordered pyrolytic graphite; IR, infrared; PPF, pyrolysed photoresist film; NBD, nitrobenzenediazonium; NP, nitrophenyl; rGO, reduced graphene oxide; STM, scanning tunneling microscopy; UHP, ultra high purity; UV, ultraviolet.

AUTHOR INFORMATION Corresponding Author * [email protected]

Tel: 64 (3) 364 2453 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments This project was supported by the Royal Society of New Zealand Marsden Fund (UOC1307). We thank Dr John Loring for use of Linkfit peak fitting software.

Page 10 of 13

REFERENCES 1. Tian, X.; Sarkar, S.; Pekker, A.; Moser, M. L.; Kalinina, I.; Bekyarova, E.; Itkis, M. E.; Haddon, R. C., Optical and Electronic Properties of Thin Films and Solutions of Functionalized Forms of Graphene and Related Carbon Materials. Carbon 2014, 72, 82-88. 2. Brooksby, P. A.; Downard, A. J., Electrochemical and Atomic Force Microscopy Study of Carbon Surface Modification via Diazonium Reduction in Aqueous and Acetonitrile Solutions. Langmuir 2004, 20, 5038-5045. 3. Pinson, J.; Podvorica, F., Attachment of Organic Layers to Conductive or Semiconductive Surfaces by Reduction of Diazonium Salts. Chem. Soc. Rev. 2005, 34, 429-439. 4. Barrière, F.; Downard, A., Covalent Modification of Graphitic Carbon Substrates by Non-Electrochemical Methods. J. Solid State Electrochem. 2008, 12, 1231-1244. 5. Ma, H.; Lee, L.; Brooksby, P. A.; Brown, S. A.; Fraser, S. J.; Gordon, K. C.; Leroux, Y. R.; Hapiot, P.; Downard, A. J., Scanning Tunneling and Atomic Force Microscopy Evidence for Covalent and Noncovalent Interactions between Aryl Films and Highly Ordered Pyrolytic Graphite. J. Phys. Chem. C 2014, 118, 5820-5826. 6. Mesnage, A.; Lefèvre, X.; Jégou, P.; Deniau, G.; Palacin, S., Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces. Langmuir 2012, 28, 11767-11778. 7. Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C., Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups. J. Am. Chem. Soc. 2009, 131, 1336-1337. 8. Sun, Z.; Kohama, S.-i.; Zhang, Z.; Lomeda, J.; Tour, J., Soluble graphene through edge-selective functionalization. Nano Res. 2010, 3, 117-125. 9. Hong, J.; Niyogi, S.; Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Amos, N.; Litvinov, D.; Berger, C.; de Heer, W. A.; Khizroev, S.; Haddon, R. C., Effect of Nitrophenyl Functionalization on the Magnetic Properties of Epitaxial Graphene. Small 2011, 7, 1175-1180. 10. Zhang, H.; Bekyarova, E.; Huang, J.-W.; Zhao, Z.; Bao, W.; Wang, F.; Haddon, R. C.; Lau, C. N., Aryl Functionalization as a Route to Band Gap Engineering in Single Layer Graphene Devices. Nano Lett. 2011, 11, 40474051. 11. Sinitskii, A.; Dimiev, A.; Corley, D. A.; Fursina, A. A.; Kosynkin, D. V.; Tour, J. M., Kinetics of Diazonium Functionalization of Chemically Converted Graphene Nanoribbons. ACS Nano 2010, 4, 1949-1954. 12. Koehler, F. M.; Jacobsen, A.; Ensslin, K.; Stampfer, C.; Stark, W. J., Selective Chemical Modification of Graphene Surfaces: Distinction Between Single- and Bilayer Graphene. Small 2010, 6, 1125-1130. 13. Zhu, H.; Huang, P.; Jing, L.; Zuo, T.; Zhao, Y.; Gao, X., Microstructure Evolution of Diazonium Functionalized graphene: A Potential Approach to Change Graphene Electronic Structure. J. Mater. Chem. 2012, 22, 2063-2068.

10


Page 11 of 13

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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14. Wang, Q. H.; Jin, Z.; Kim, K. K.; Hilmer, A. J.; Paulus, G. L. C.; Shih, C.-J.; Ham, M.-H.; SanchezYamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kong, J.; Jarillo-Herrero, P.; Strano, M. S., Understanding and Controlling the Substrate Effect on Graphene ElectronTransfer Chemistry via Reactivity Imprint Lithography. Nat. Chem. 2012, 4, 724-732. 15. Sharma, R.; Baik, J. H.; Perera, C. J.; Strano, M. S., Anomalously Large Reactivity of Single Graphene Layers and Edges toward Electron Transfer Chemistries. Nano Lett. 2010, 10, 398-405. 16. Huang, P.; Jing, L.; Zhu, H.; Gao, X., Diazonium Functionalized Graphene: Microstructure, Electric, and Magnetic Properties. Acc. Chem. Res. 2012, 46, 43-52. 17. Niyogi, S.; Bekyarova, E.; Itkis, M. E.; Zhang, H.; Shepperd, K.; Hicks, J.; Sprinkle, M.; Berger, C.; Lau, C. N.; deHeer, W. A.; Conrad, E. H.; Haddon, R. C., Spectroscopy of Covalently Functionalized Graphene. Nano Lett. 2010, 10, 4061-4066. 18. Wood, B. C.; Ogitsu, T.; Otani, M.; Biener, J., First-Principles-Inspired Design Strategies for GrapheneBased Supercapacitor Electrodes. J. Phys. Chem. C 2013, 118, 4-15. 19. Wu, B.; Geng, D.; Guo, Y.; Huang, L.; Xue, Y.; Zheng, J.; Chen, J.; Yu, G.; Liu, Y.; Jiang, L.; Hu, W., Equiangular Hexagon-Shape-Controlled Synthesis of Graphene on Copper Surface. Adv. Mater. 2011, 23, 35223525. 20. Cummings, A. W.; Duong, D. L.; Nguyen, V. L.; Van Tuan, D.; Kotakoski, J.; Barrios Vargas, J. E.; Lee, Y. H.; Roche, S., Charge Transport in Polycrystalline Graphene: Challenges and Opportunities. Adv. Mater. 2014, 26, 50795094. 21. Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K., Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308. 22. Greenwood, J.; Phan, T. H.; Fujita, Y.; Li, Z.; Ivasenko, O.; Vanderlinden, W.; Van Gorp, H.; Frederickx, W.; Lu, G.; Tahara, K.; Tobe, Y.; Uji-i, H.; Mertens, S. F. L.; De Feyter, S., Covalent Modification of Graphene and Graphite Using Diazonium Chemistry: Tunable Grafting and Nanomanipulation. ACS Nano 2015, 9, 5520-5535. 23. Bekyarova, E.; Sarkar, S.; Niyogi, S.; Itkis, M. E.; Haddon, R. C., Advances in the Chemical Modification of Epitaxial Graphene. J. Phys. D: Appl. Phys. 2012, 45, 154009. 24. Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S., Raman Spectroscopy in Graphene. Physics Reports 2009, 473, 51-87. 25. Jayasundara, D. R.; Cullen, R. J.; Colavita, P. E., In Situ and Real Time Characterization of Spontaneous Grafting of Aryldiazonium Salts at Carbon Surfaces. Chem. Mater. 2013, 25, 1144-1152. 26. Baranowski, J. M.; Mozdzonek, M.; Dabrowski, P.; Grodecki, K.; Osewski, P.; Kozlowski, W.; Kopciuszynski, M.; Strupinski, W., Observation of Electron-Phonon

Couplings and Fano Resonances in Epitaxial Bilayer Graphene. Graphene 2013, 2, 115-120. 27. Tang, T.-t.; Zhang, Y.; Park, C.-h.; Geng, B.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Louie, S. G.; Shen, Y. R.; Wang, F., A Tunable Phonon-Exciton Fano System in Bilayer Graphene. Nat. Nanotechnol. 2010, 5, 32-6. 28. Cullen, R. J.; Jayasundara, D. R.; Soldi, L.; Cheng, J. J.; Dufaure, G.; Colavita, P. E., Spontaneous Grafting of Nitrophenyl Groups on Amorphous Carbon Thin Films: A Structure–Reactivity Investigation. Chem. Mater. 2012, 24, 1031-1040. 29. Madec, L.; Robert, D.; Moreau, P.; BayleGuillemaud, P.; Guyomard, D.; Gaubicher, J., Synergistic Effect in Carbon Coated LiFePO4 for High Yield Spontaneous Grafting of Diazonium Salt. Structural Examination at the Grain Agglomerate Scale. J. Am. Chem. Soc. 2013, 135, 11614-11622. 30. Murphy, D. M.; Cullen, R. J.; Jayasundara, D. R.; Scanlan, E. M.; Colavita, P. E., Study of the Spontaneous Attachment of Polycyclic Aryldiazonium Salts onto Amorphous Carbon Substrates. RSC Advances 2012, 2, 6527-6534. 31. Pandurangappa, M.; Ramakrishnappa, T.; Compton, R. G., Functionalization of Glassy Carbon Spheres by Ball Milling of Aryl Diazonium Salts. Carbon 2009, 47, 2186-2193. 32. Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreery, R. L., Determination of the Structure and Orientation of Organic Molecules Tethered to Flat Graphitic Carbon by ATR-FT-IR and Raman Spectroscopy. Anal. Chem. 2006, 78, 3104-3112. 33. Mahmoud, A. M.; Bergren, A. J.; McCreery, R. L., Derivatization of Optically Transparent Materials with Diazonium Reagents for Spectroscopy of Buried Interfaces. Anal. Chem. 2009, 81, 6972-6980. 34. Liu, Y.-C.; McCreery, R. L., Reactions of Organic Monolayers on Carbon Surfaces Observed with Unenhanced Raman Spectroscopy. J. Am. Chem. Soc. 1995, 117, 11254-11259. 35. Chamoulaud, G.; Bélanger, D., Spontaneous Derivatization of a Copper Electrode with in Situ Generated Diazonium Cations in Aprotic and Aqueous Media. J. Phys. Chem. C 2007, 111, 7501-7507. 36. Chen, Q.; Haq, S.; Frederick, B. G.; Richardson, N. V., Adsorption of Nitrobenzene and Some Simple Derivatives on the Cu(110) Surface. Surf. Sci. 1996, 368, 310317. 37. Syomin, D.; Wang, J.; Koel, B. E., Monolayer and Multilayer Films of Nitrobenzene on Au(111) Surfaces: Bonding and Geometry. Surf. Sci. 2001, 495, L827-L833. 38. Itoh, T.; McCreery, R. L., In Situ Raman Spectroelectrochemistry of Electron Transfer between Glassy Carbon and a Chemisorbed Nitroazobenzene Monolayer. J. Am. Chem. Soc. 2002, 124, 10894-10902. 39. Richner, G.; van Bokhoven, J. A.; Neuhold, Y.-M.; Makosch, M.; Hungerbuhler, K., In Situ Infrared

11



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 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 13

Monitoring of the Solid/Liquid Catalyst Interface During the Three-Phase Hydrogenation of Nitrobenzene Over Nanosized Au on TiO2. PCCP 2011, 13, 12463-12471. 40. Chehimi, M. M.; Lamouri, A.; Picot, M.; Pinson, J., Surface Modification of Polymers by Reduction of Diazonium Salts: Polymethylmethacrylate as an Example. J. Mater. Chem. C 2014, 2, 356-363. 41. Yu, H.-Z.; Ye, S.; Zhang, H.-L.; Uosaki, K.; Liu, Z.F., Molecular Orientation and Electrochemical Stability of Azobenzene Self-Assembled Monolayers on Gold: An InSitu FTIR Study. Langmuir 2000, 16, 6948-6954. 42. Yu, H.-Z.; Zhang, H.-L.; Liu, Z.-F.; Ye, S.; Uosaki, K., Monitoring Electron Transfer in an Azobenzene SelfAssembled Monolayer by in Situ Infrared Reflection Absorption Spectroscopy. Langmuir 1998, 14, 619-624. 43. Dasgupta, S.; Wang, D.; Kübel, C.; Hahn, H.; Baumann, T. F.; Biener, J., Dynamic Control Over Electronic Transport in 3D Bulk Nanographene via Interfacial Charging. Adv. Funct. Mater. 2014, 24, 34943500. 44. Lehr, J.; Williamson, B. E.; Downard, A. J., Spontaneous Grafting of Nitrophenyl Groups to Planar Glassy Carbon Substrates: Evidence for Two Mechanisms. J. Phys. Chem. C 2011, 115, 6629-6634. 45. Brooksby, P. A.; Downard, A. J., Multilayer Nitroazobenzene Films Covalently Attached to Carbon. An AFM and Electrochemical Study. J. Phys. Chem. B 2005, 109, 8791-8798. 46. Abiman, P.; Wildgoose, G. G.; Compton, R. G., Investigating the Mechanism for the Covalent Chemical Modification of Multiwalled Carbon Nanotubes using Aryl Diazonium Salts. Inter. J. Electrochem. Sci. 2008, 3, 104-117. 47. Adenier, A.; Bernard, M.-C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F., Covalent Modification of Iron Surfaces by Electrochemical Reduction of Aryldiazonium Salts. J. Am. Chem. Soc. 2001, 123, 4541-4549. 48. Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K., Nitrophenyl Groups in Diazonium-Generated Multilayered Films: Which are Electrochemically Responsive? Langmuir 2010, 26, 1081210821. 49. Chua, C. K.; Pumera, M.; Rulíšek, L., Reduction Pathways of 2,4,6-Trinitrotoluene: An Electrochemical and Theoretical Study. J. Phys. Chem. C 2012, 116, 4243-4251.

12


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 34 35 36 37 38 39 40 41 42


HOPG NO 2

e-

NO 2

NO 2

e-

NO 2

eFLG

 (NO2) / 10-9 mol.cm-2

Page 13 of 13

2 potl up vs offset current for up

1

0 0

10 20 30 40 50 60 70

Time / h


Spontaneous Modification of Free-Floating Few-Layer Graphene by

Recommend Documents