Synthesis of Carboxylated-Graphenes by the ... - ACS Publications

Synthesis of Carboxylated-Graphenes by the Kolbe−Schmitt Process Alex Yong Sheng Eng,† Zdeněk Sofer,‡ David Sedmidubský,‡ and Martin Pumera*,† †

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Graphene oxide is an oxidized form of graphene containing a large variety of oxygen groups. Although past models have suggested carboxylic acids to be present in significant amounts, recent evidence has shown otherwise. Toward the production of carboxyl-graphene, a synthetic method is presented herein based on the Kolbe− Schmitt process. A modified procedure of heating graphite oxide in the presence of a KOH/CaO mixture results in up to 11 atom % of carboxylic groups. The graphite oxide starting material and reaction temperature were investigated as two important factors, where a crumpled morphology of graphite oxide flakes and a lower 220 °C temperature preferentially led to greater carboxyl functionalization. Successful carboxylation caused a band gap opening of ∼2.5 eV in the smallest carboxyl−graphene particles, which also demonstrated a yellow fluorescence under UV light unseen in its counterpart produced at 500 °C. These results are in good agreement with theoretical calculations showing band gap opening and spin polarization of impurity states. This demonstrates the current synthetic process as yet another approach toward tuning the physical properties of graphene. KEYWORDS: graphene, graphene oxide, functionalization, Kolbe−Schmitt process, fluorescence

G

raphene is a sp2-hybridized carbon allotrope and may simply be described as an atomically thin layer of graphite. As a pristine material, it is chemically inert and can thus be applied as protective barriers and corrosionresistant coatings over various surfaces.1−4 Graphene oxide and its bulk form graphite oxide (GO) on the other hand contain a large variety of functional groups such as epoxyls, carbonyls, hydroxyls, quinones,5,6 and sometimes also a small quantity of carboxylic acids,7,8 which together confer a high degree of chemical reactivity in contrast to pure graphene. Engineering the surface functionalities of graphene materials consequently offers benefits such as the control and modulation of related electronic and magnetic properties, including their electrical conductivity, magnetism, band gap, or fluorescence as one desires for intended applications.9−12 Several approaches may be employed in principle to achieve surface group modifications: with GO as starting material, the most straightforward method would be to attempt the selective removal of unwanted moieties.13 However, as GO is an amorphous and nonstoichiometric material,5 considerable difficulty exists in controlling both the exact type and amount of each surface moiety. Alternative means include substitution of a pre-existing functionality,14 or the introduction of a desired group directly onto a pristine15 or prior-reduced16 graphene © 2017 American Chemical Society

surface. In particular for carboxylic groups, the abovementioned examples typically result in low carboxyl contents either due to limited amounts of edge sites or the inertness of the graphitic basal plane.5,7,8,16 In this article, we report on the synthesis and properties of carboxyl-graphene prepared by utilizing existing −OH groups (particularly phenols) to introduce carboxyls. Graphite oxide is treated with an established carboxylation reaction for the general conversion of phenols to hydroxybenzoic acids. Known as the Kolbe−Schmitt reaction which typically involves the heating of a phenoxide salt under carbon dioxide at elevated pressure,17,18 it is used in industries today such as for the large scale production of salicylic acid.19 Interestingly for our modified procedure with GO, carbon dioxide can instead be produced in situ upon thermal treatment, where it is released from GO as a result of degradation of its large number of oxygen functionalities.5,20−25 Two factors were investigated, namely the type of GO starting material and the reaction temperature. Synthesis of the carboxyl-graphene was attempted Received: November 17, 2016 Accepted: January 12, 2017 Published: January 17, 2017 1789

DOI: 10.1021/acsnano.6b07746 ACS Nano 2017, 11, 1789−1797

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ACS Nano Scheme 1. Kolbe−Schmitt Reactiona

a

(a) Classical process for the carboxylation of phenol. (b) Modified procedure for carboxylation of graphite oxide with inclusion of 10 wt % calcium oxide and in situ CO2 generation.

Figure 1. (a) Structure model of carboxylated graphene sheet (C8COOH) used for DFT calculations. (b) Density of states of C8COOH including spin polarized midgap impurity bands. (spin-down states depicted with negative sign) The electron density (purple) associated with spin-up channel of midgap states is shown in the inset.

uct.29 In our case with GO however (Scheme 1b), the method is modified to include 10 wt % calcium oxide for the purpose to trap carbon dioxide that is outgassed during the thermal reduction of GO when heated above 160 °C. This in situ outgassing of carbon dioxide occurs together with carbonates present as impurities in KOH as the main reagent of the Kolbe−Schmitt reaction. The elevated temperatures then result in a thermal exfoliation/deoxygenation process that not only releases large amounts of water vapor, carbon dioxide, and carbon monoxide,20,22,24 but also smaller amounts of various organic volatiles.23,25 In particular, Barroso-Bujans et al. reported that the reduction of GO starts above 150 °C and continues to 220 °C where the main reduction process was seen to occur.20 On the basis of their X-ray diffraction data, the peak due to graphitic structures then reached a maximum at 500 °C.20 Similar observations were made by Eigler and coworkers with peak release of carbon dioxide seen occurring at 153 °C.22 Hence in this regard, we employ a two-step heating procedure in our synthesis: with the first 3 h heating at 160 °C to achieve the outgassing of carbon dioxide crucial for carboxyl formation, and the second 3 h annealing step at either 220 or 500 °C to investigate the removal extents of other unwanted inherent oxygen functionalities. This procedure is applied to the

on GO starting materials produced from three different oxidations, namely the Staudenmaier,26 Hofmann,27 and Hummers28 methods, showing differing effectiveness for carboxylic group formation. Another crucial factor was the reaction temperature which affected the removal of other unwanted oxygen groups, while retaining the desired carboxyls. Consequently, creation of carboxyl groups was verified from infrared and X-ray photoelectron spectra, while the structural morphologies were tracked with scanning electron microscopy and Raman spectroscopy. Finally, we observe major differences in electrochemical and optical properties, such as fluorescence only occurring with a significantly high carboxylation extent.

RESULTS AND DISCUSSION In this study, a modified version of the Kolbe−Schmitt reaction is performed to produce carboxyl-graphenes from graphite oxide. Scheme 1a illustrates the classical process using phenol, which is first reacted with hydroxide to give the phenolate salt. This is subsequently heated under pressure with carbon dioxide to yield a hydroxy acid after acid workup. Earlier works have determined a preference for ortho-substitution to yield salicylic acid with the use of sodium hydroxide, while potassium hydroxide preferentially results in the para-substituted prod1790


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CO stretch in G-HO-220 and G-HU-220 is most prominent at the positions of 1705 and 1708 cm−1, respectively. Hence, we can derive two important requirements for the formation of carboxylic groups: a lower reaction temperature of 220 °C and using either the Hofmann or Hummers GO as starting material which favorably resulted in carboxyl functionalization. Both GHO-220 and G-HU-220 also have the most intense aromatic CC stretches at 1560 cm−1 and C−O bands at 1200 cm−1.32 There is also a lack of −OH stretches associated with carboxylic acids in all but G-HO-220 and G-HU-220 with very broad bands about 3200 cm−1. Further chemical information was elucidated from high resolution XPS, where the carbon 1s signal was deconvoluted to reveal carbon species at varying binding energies (Figure 3). All spectra were dominated by the primary sp2 carbon CC peak at 284.5 eV and tapered toward higher binding energies, similar to previous thermally treated graphenes.33 In agreement with FTIR, the spectra of both G-ST graphenes and G-HO-500 did not demonstrate any notable amounts of oxygen functionalities, with carboxylic carbons at ca. 288.7 eV (labeled O−CO) comprising approximately only about 4 atom % in each graphene (see Supporting Information Table S1). G-HU-500 had a slightly larger amount at 6.4 atom %, which may be explained by the small number of pre-existing carboxyl groups found only in the Hummers GO starting material.7 The highest carboxyl compositions were found instead in G-HO-220 and GHU-220 which appear as obvious peaks at the representative 288.7 eV, which accounts for approximately 11 atom % of all carbon in both graphenes. The larger presence of carboxyl functionalities in G-HO-220 and G-HU-220 was also reflected in the wide-scan XPS (Figure S1), in which the oxygen composition obtained was 21.5 atom % and 17.2 atom %, respectively, for both carboxylated graphenes (Table 1). Correspondingly, their carbon-to-oxygen ratios remained low as compared to the other graphenes. It was also noted that all graphenes produced at 500 °C showed higher C/O ratios than their counterparts at 220 °C, as a result of greater outgassing of oxygen groups at elevated temperature. 20,22,23 Further elemental combustion analysis (Table S2) showed interestingly that G-HO-220 distinctly stood out with the highest oxygen composition of 27.73 atom % in contrast to G-HU-220 with 16.20 atom %. This likely indicates that the oxygen groups in G-HO-220 are well distributed throughout the bulk material, while they may be limited to the surface for the case of G-HU220 when we consider the surface sensitivity of the XPS technique. Nevertheless, mapping by energy-dispersive X-ray spectroscopy shows that oxygen is dispersed over all graphene platelets (Figure S2). Variations in the structure of the graphenes were subsequently investigated by Raman spectroscopy. Figure 4 compares the differences of each graphite oxide depending on the temperature of the carboxyl synthesis. All spectra display the D (disorder) phonon mode at 1350 cm−1 due to sp3hybridized defect sites and the G (graphitic) band at approximately 1580 cm−1 characteristic of the pristine sp2 lattice.34,35 Comparing the intensities of the D-band to Gband (ID/IG ratio) thus identifies the relative amount of disorder from oxygen functionalities and other sp3-defects within the graphenes (Figure 4d). First, all graphenes obtained at 500 °C had higher ID/IG ratios than their counterparts at 220 °C due to increased defects formed during the outgassing process,20,22,23,25 corroborating with their higher XPS C/O ratios. Next, both G-ST graphenes were then seen to have

Staudenmaier, Hofmann, and Hummers GOs, which are respectively abbreviated as G-ST, G-HO, and G-HU. The temperature of the second 3 h annealing step is further indicated as the suffix. The electronic structure of carboxylated graphenes was modeled by DFT using Wien2k code for crystalline solids by assuming a superstructure with a single COOH group per eight carbon atoms in graphene sheet (see Figure 1a). The presence of carboxyl groups attached to in-plane atoms causes opening of a band gap ΔEg = 1.5 eV with a deep level band inside the gap. As seen from the inset of Figure 1b, the impurity states have a predominant character of C-2pz orbitals perpendicular to the graphene layer located on the nearest neighbor C(NN) of the carbon atom C(D) to which the carboxyl group is attached. The localization of the πp states is clearly due to blocking of the coherent πp overlap by C(D) atoms, which are distinctly displaced above the layer and use the pz orbital to form a bond toward carboxyl. Moreover the spin-polarized calculation revealed that the spin-up and spin-down impurity states at the Fermi level are split by exchange interaction yielding a spin-flip gap of 0.75 eV. It should be noted that such an effect should be generally expected for any impurities invoking a perpendicular displacement of in-plane carbon atoms and thus disrupting the sp2 bonding pattern combined with additional πp interaction. In fact the spin polarized impurity state can be viewed as a single electron occupying a molecular orbital centered on three C(NN) atoms adjacent to C(D) atom hosting a functional group. In the next step we analyzed the characteristic chemical species present in the prepared materials by FT-IR spectroscopy (Figure 2). The main signal of interest is the dominant CO stretching mode which falls at 1715 cm−1 for a ketone.30,31 CO stretches in carboxylic acids have a bathochromic shift to ca. 1705 cm−1. Both G-ST graphenes and G-HO-500 have noticeably absent CO signals. In contrast, G-HU-500 only has a minute CO signal, while the

Figure 2. ATR-FTIR spectra of carboxylated-graphenes prepared from the Kolbe−Schmitt reaction on graphite oxide materials under different conditions. 1791


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Figure 3. High-resolution carbon-1s X-ray photoelectron spectra of carboxylated-graphenes prepared from the Kolbe−Schmitt reaction on graphite oxide materials under different conditions.

The observations from the Raman spectra of G-ST graphenes agree with the morphology seen from SEM in Figure 5, where they exist as large stacked platelets with parallel sheet edges suggesting little damage either by the initial oxidation or the subsequent carboxyl functionalization treatment. Sheet crumpling and detachment were mostly limited only to exposed external layers. The Staudenmaier oxidation procedure typically results in the least structural damage to graphite and low oxidation extent, resulting in G-ST graphenes having the lowest carboxyl functionalization. In contrast, both G-HO and G-HU graphenes demonstrated extensive crumpling and reduction of lateral sheet size, which as seen earlier significantly improved the carboxyl functionalization extent. While no major structural difference can be seen for graphenes reacted at 220 °C versus 500 °C, we can expect the higher annealing temperature to be less conducive for carboxyl formation as more oxygen groups, particularly −OH, are outgassed.20 Therefore, the structural morphology conferred by the initial GO oxidation procedures plays an important role for subsequent formation of carboxylic groups. We also observed that as a likely result of the increased carboxyl functionalization, only G-HO-220 and both G-HU graphenes produced stable dispersions in N,N-dimethylforma-

Table 1. Surface Atomic Compositions of Various Graphenes Treated by the Kolbe−Schmitt Reaction Based on Wide-Scan X-ray Photoelectron Spectroscopy material

C/O ratio

C 1s atom %

O 1s atom %

G-ST-220 G-ST-500 G-HO-220 G-HO-500 G-HU-220 G-HU-500

8.69 9.82 3.64 9.59 4.81 8.08

89.7 90.8 78.5 90.1 82.8 89.0

10.3 9.2 21.5 9.4 17.2 11.0

Cl 2p atom %

0.5

significantly lower I D /I G ratios, and also taking into consideration the sharp G-band we may conclude that they are the least defective and oxidized. The G-ST materials also show a small 2D (also termed G′) band at about 2680 cm−1, and this signal arises from a double-phonon mode that is most commonly observed in graphene without defects, therefore suggesting the existence of some pristine graphitic sheets that are not present in the other materials. Its symmetrical peak shape is also indicative of turbostratic graphene consisting of misaligned layers.35 1792


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Figure 4. Raman spectra of carboxylated-graphenes prepared by the Kolbe−Schmitt reaction at different temperatures using the (a) Staudenmaier, (b) Hofmann, and (c) Hummers graphite oxides, respectively. λexcitation = 514.5 nm (d) Average ratios of the Raman D and G bands for carboxylated-graphenes. Error bars represent the standard deviation from five measurements.

220 and G-HO-500 obtained after a final centrifugation step at 9000 rpm for 10 min. The peak absorbance of the G-HO-220 supernatant was seen at 270 nm with a shoulder at approximately 470 nm, and appears as a yellow-brown solution (Figure 6d). The G-HO-500 supernatant was colorless and showed no absorption across the UV−visible region. On the basis of Tauc’s method,38 the optical band gap has been estimated for various graphene-type materials.39,40 By extrapolating the linear region of Tauc’s plot to the horizontal axis in Figure 6b, we find the G-HO-220 supernatant to have an estimated band gap of about 2.5 eV while the precipitate collected after the first 1000 rpm centrifugation step (Figure S5b) gave a band gap of 1.4 eV. Neither supernatant nor precipitate of G-HO-500 (Figure S5d) showed discernible band gaps with linear intercepts close to zero. Interestingly, the GHO-220 supernatant also showed a yellow fluorescence when irradiated with a 365 nm UV light source as shown in Figure 6f, and its fluorescence spectra (Figure 6c) displayed a strong emission at approximately 540 nm when excited at 470 nm. Its 1000 rpm precipitate also showed a fluorescence signal but with a markedly lower intensity. This is again contrasted to G-HO500 which demonstrated no fluorescence. Hence, it is clear that

mide. This was unlike the other less functionalized graphenes which agglomerated and settled fully after just 3 days of standing (Figure S4). Considering the known positive effect of edge defects on the observed heterogeneous electron transfer (HET) at graphene surfaces,36,37 a short cyclic voltammetry experiment was performed for the ferro/ferricyanide redox probe (Figure S3). As expected, both G-ST graphenes showed the slowest HET with large peak-to-peak separations (ΔEp) of the ferro/ferricyanide probe, at 0.32 and 0.38 V for G-ST-220 and G-ST-500, respectively. This was poorer than the bare glassy carbon electrode at 0.30 V, while the HET at G-HO and G-HU graphenes were faster with ΔEp varying in a broad range from 0.26 to 0.17 V due to the crumpling of the sheets that resulted in more edge sites that facilitated electron transfer. Finally, G-HO-220 and G-HO-500 were chosen for comparison of their optical properties, as they exhibited the greatest differences in terms of carboxyl content based on XPS and also their defect density from Raman spectroscopy. As all prepared graphene dispersions may be expected to contain a large distribution of sheet sizes, gradient centrifugation was first performed to separate the materials (Figure 6d,e). Figure 6a displays the absorption spectra of the supernatants of G-HO1793


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Figure 5. Scanning electron micrographs of carboxylated-graphenes prepared from the Kolbe−Schmitt reaction on graphite oxide materials under different conditions.

Figure 6. Comparison of optical properties for carboxylated graphenes G-HO-220 and G-HO-500 in N,N-dimethylformamide. (a) UV−vis absorbance spectra of supernatant solutions obtained after final step of gradient centrifugation at 9000 rpm for 10 min. (b) Tauc plots of carboxylated graphenes. (c) Fluorescence spectra of supernatant solutions obtained after centrifugation at 9000 rpm for 10 min (denoted Sp) and dispersions of the precipitate after initial centrifugation at 1000 rpm for 10 min (denoted Ppt); λexcitation = 470 nm. Optical images of redispersed (d) G-HO-220 and (e) G-HO-500 precipitates following gradient centrifugation at increasing speeds of 1000, 3000, 6000, and 9000 rpm (left to right). Rightmost solutions are supernatants from the final 9000 rpm centrifugation step. Suspensions of (f) G-HO-220 and (g) G-HO-500 under UV light irradiation at 365 nm.

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potassium chlorate was added over 30 min. The reaction flask was loosely capped to allow the escape of chlorine dioxide gas. The mixture was continuously stirred for 96 h at room temperature and then poured into 3 L of deionized water. After decantation, the graphite oxide was redispersed in 5% hydrochloric acid, before a second decantation from hydrochloric acid. The GO was repeatedly centrifuged and redispersed until a negative reaction for chloride and sulfate ions (with Ba(NO3)2 and AgNO3) was observed. Graphite oxide slurry was finally dried in a vacuum oven at 60 °C for 48 h before further use. Synthesis Procedure of Hofmann Graphite Oxide.27 The graphite oxide prepared by the Hofmann method was termed HO-GO. Concentrated sulfuric acid (87.5 mL) and nitric acid (27 mL) were added to a reaction flask containing a magnetic stir bar. The mixture was then cooled at 0 °C, and graphite (5 g) was added. The mixture was vigorously stirred to avoid agglomeration and to obtain a homogeneous dispersion. While keeping the reaction flask at 0 °C, potassium chlorate (55 g) was slowly added to the mixture in order to avoid a sudden increase in temperature and the consequent formation of explosive chlorine dioxide gas. Upon the complete dissolution of the potassium chlorate, the reaction flask was then loosely capped to allow the escape of the evolved gas and the mixture was continuously stirred vigorously for 96 h at room temperature. On completion of the reaction, the mixture was poured into deionized water (3 L) and decanted. The graphite oxide was first redispersed in HCl (5%) solutions to remove sulfate ions and then repeatedly centrifuged and redispersed in deionized water until all chloride and sulfate ions were removed (negative reaction with AgNO3 and Ba(NO3)2). The graphite oxide slurry was then dried in a vacuum oven at 50 °C for 48 h before use. Synthesis Procedure of Graphite Oxide with the Modified Hummers Method.28 The synthesis of HU-GO was based on a method reported previously: 115 mL of sulfuric acid (98%) was initially cooled to 0 °C with the subsequent addition of 5 g of graphite and 2.5 g of NaNO3. While the mixture was vigorously stirred, 15 g of KMnO4 was added over a period of 2 h. The reaction mixture was then removed from the ice bath and stirred at room temperature for 4 h. The reaction mixture was then heated to 35 °C for 30 min, and poured into 250 mL of deionized water with heating to 70 °C. After 15 min the mixture was poured into 1 L of deionized water. Unreacted KMnO4 was decomposed with 10% hydrogen peroxide. The reaction mixture was then decanted and repeatedly centrifuged and redispersed until a negative reaction for sulfate ions (with Ba(NO3)2) was achieved. The graphite oxide slurry was then dried in a vacuum oven at 60 °C for 48 h before further use. Synthesis of Carboxylated Graphenes by the Modified Kolbe−Schmitt Process. A 0.5 g portion of GO was fused with 15 g of the KOH/CaO mixture (10 wt % CaO) for 3 h at 160 °C and subsequently for 3 h at 220 or 500 °C. Heating at 500 °C was performed under Ar atmosphere. A silver crucible with reaction mixture was covered with a silver lid and placed in a muffle furnace and heated at 160 °C for 3 h. Subsequently the temperature was increased to 220 °C, and the reaction mixture was heated for next 3 h. For heating at 500 °C, the reaction mixture was placed in quartz tube and heated at 500 °C for 3 h under argon atmosphere (500 mL/min). The reaction mixture was leached with water and removed by suction filtration and repeatedly washed with water. Subsequently, the separated carboxylated graphenes were dispersed in 100 mL of 10 wt % hydrochloric acid, ultrasonicated for 5 min, separated by suction filtration, and again washed repeatedly with water. Finally, the product was dried in vacuum oven for 48 h at 50 °C. Materials Characterization. Fourier-transform infrared (FTIR) spectra were obtained in attenuated total reflectance (ATR) mode using a PerkinElmer Spectrum 100 spectrometer, with samples directly compressed onto a diamond/ZnSe crystal. X-ray photoelectron spectroscopy was performed using a Phoibos 100 spectrometer (SPECS) with a magnesium Kα X-ray source for both survey and high resolution carbon 1s scans. The prepared graphenes were compressed onto conductive carbon tape and mounted on aluminum sample holders. Combustible elemental analysis (CHNS-O) was performed

the carboxylation of G-HO-220 resulted in its electronic properties with an optical band gap and fluorescence. The range of optical band gaps in carboxylated G-HO-220 also appears to be dependent on their size. Indeed the 1.4 eV band gap observed with the centrifugation precipitate with appreciably larger particles corresponds to that obtained by DFT for an infinite carboxylated plane (Figure 1), while the ultimate band gap value of 2.5 eV appears to be associated with nanosized flakes with a large fraction of carboxyls located on their edges. Since the synthesis was performed in potassium hydroxide it is unlikely to correlate the luminescence with any debris present on graphene oxide surfaces. The use of hydroxide has been reported as an effective method for removal of graphite oxidative debris related to the luminescence of graphene oxide derivates.41 Moreover, most recent detailed work on the optical properties of graphene oxide indicates a negligible influence of the oxidative debris on the optical properties of graphene oxide and related materials.42

CONCLUSIONS The synthesis of carboxyl-functionalized graphene is presented in this report, using a modification of the Kolbe−Schmitt process. Heating of graphite oxide (GO) in the presence of a KOH/CaO mixture results in up to approximately 11 atom % of carboxylic groups. However, the choice of GO starting material and reaction temperature were found to be important factors, where the crumpled morphology of the Hofmann and Hummers GOs resulted in better efficacy and a 220 °C reaction temperature was preferable due to reduced outgassing of inherent oxygen groups. Interestingly, successful carboxylation of the Hofmann GO at 220 °C demonstrated opening of an optical bandgap and also exhibited a yellow fluorescence under UV irradiation, which were unseen in its counterpart produced at 500 °C. These results are in good agreement with the theoretical calculation showing band gap opening as well as spin polarization indicating magnetic ordering. While recent evidence has suggested that carboxyl contents are very low, if present at all, in as-produced GO, the method shown provides a viable option for obtaining a graphene material with significantly greater carboxyl functionalization. Future efforts could also consider the use of an external CO2 supply which might further increase the functionalization extent. Carboxylation from the modified Kolbe−Schmitt process adds to our assortment of methods to manipulate the surface functionalization of graphenes, allowing for modulation of electronic properties for optical and/or magnetic applications. METHODS Materials. Sulfuric acid (98%, p.a.), nitric acid (68%, p.a.), potassium chlorate (99%, p.a.), potassium permanganate (99%, p.a), sodium nitrate (99.5%, p.a.), hydrogen peroxide (30%, p.a.), potassium hydroxide (96%, p.a.), calcium oxide (99%, p.a.), hydrochloric acid (35%, p.a.), barium nitrate (99%, p.a.), silver nitrate (99.5%, p.a.) were obtained from PENTA, Czech Republic. Graphite microparticles (2− 15 μm, 99.9995%) were obtained from Alfa Aesar, Germany. Argon of 99.999% purity was obtained from SIAD, Czech Republic. Glassy carbon (GC) working electrodes (3 mm diameter), a platinum counter electrode (Pt), and an Ag/AgCl reference electrode (1 M KCl) were purchased from CH Instruments, USA. Deionized water with a resistivity of 18.2 MΩ cm was employed throughout. Synthesis Procedure of Staudenmaier Graphite Oxide.26 An 87.5 mL aliquot of sulfuric acid (98%) and 27 mL of nitric acid (98%) were first cooled to 0 °C, before the addition of 5 g of graphite to the acid mixture. The mixture was stirred vigorously while 55 g of 1795


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ACS Nano with a PE 2400 series II CHNS/O Analyzer (PerkinElmer, USA). In CHN operating mode, the instrument employs a classical combustion principle to convert the sample elements to simple gases (CO2, H2O, and N2). The PE 2400 analyzer automatically performs combustion and reduction, homogenization of product gases, then separation and detection. A microbalance MX5 (Mettler Toledo) was used for precise weighing of samples (1.5−2.5 mg per single sample analysis). The accuracy of CHN determination is better than 0.30% abs. Internal calibration was performed using N-fenyl urea. Raman spectra were recorded from a confocal micro-Raman LabRam HR instrument (Horiba Scientific) with a CCD detector in backscattering geometry, using a 514.5 nm Ar excitation laser and a 100× objective lens mounted on an Olympus microscope. The instrument was calibrated with a silicon reference giving a signal at 520 cm−1. Materials were drop-casted onto silicon wafers and dried before analysis. Morphological imaging by scanning electron microscopy was performed from a JEOL 7600F field-emission scanning electron microscope (JEOL, Japan) in gentle-beam mode at a 2 kV accelerating voltage, while 15 kV was used when obtaining energy dispersive X-ray spectra. Gradient centrifugation was performed with an initial 4 mL aliquot of a 1.0 mg mL−1 graphene dispersion in DMF, at increasing speeds from 1000 rpm, 3000 rpm, 6000 rpm, to 9000 rpm for 10 min each. The supernatant was removed after each centrifugation step, and the residue was redispersed in 4 mL of fresh solvent. UV−vis spectra were obtained from a Cary 100 Bio spectrophotometer with dispersions placed in quartz cuvettes, and fluorescence spectroscopy performed on a Cary Eclipse fluorescence spectrometer with high photomultiplier tube voltage. Electrochemistry Studies. Cyclic voltammetry was performed using a three-electrode configuration on an Autolab PGSTAT 101 electrochemical analyzer (Eco Chemie, The Netherlands). All electrochemical potentials are with respect to the Ag/AgCl (1 M KCl) reference electrode, employing a fixed scan rate of 0.1 V s−1 and measured at room temperature. A 50 mM PBS supporting electrolyte at pH 7.2 was used with 10 mM of the ferro/ferricyanide probe. The solution was purged with nitrogen prior to experiments. GC electrodes were renewed before every measurement by polishing with a 0.05 μm alumina/water slurry on a polishing pad and then rinsed with deionized water. Graphenes were immobilized onto GC surfaces by transferring 1.0 μL aliquots (1.0 mg mL−1 in DMF) of each suspension. This transfer was repeated three times to give a loading of 42.4 μg cm−2 per electrode and was done to obtain uniform layers and to prevent influence from possible thin-layer effects.43 DFT Calculations. The electronic structure of carboxylated graphene represented by a supercell corresponding to a formula C8COOH was calculated within density functional theory (DFT) using the APW+lo basis set and generalized gradient approximation (GGA, PBE96 parametrization scheme) for the exchange correlation potential as implemented in the Wien2k software package. The twodimensional character of the functionalized graphene sheet was approximated by inserting a 25 Å thick vacuum region between the individual layers. The calculation was performed both with and without the inclusion of spin polarization. The plane wave cutoff energy of 394 eV and the tetrahedron method with the k-mesh 10 × 10 × 1 were used.

supernatant and precipitate dispersions obtained from gradient centrifugation of G-HO-220 and G-HO-500 (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Alex Yong Sheng Eng: 0000-0001-9577-1681 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS M.P. acknowledges a Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education, Singapore. Z.S. and D.S. were supported by Czech Science Foundation (GACR No. 15-09001S and 16-05167S). REFERENCES (1) Nine, M. J.; Cole, M. A.; Tran, D. N. H.; Losic, D. Graphene: A Multipurpose Material for Protective Coatings. J. Mater. Chem. A 2015, 3, 12580−12602. (2) Krishnamurthy, A.; Gadhamshetty, V.; Mukherjee, R.; Natarajan, B.; Eksik, O.; Shojaee, S. A.; Lucca, D. A.; Ren, W.; Cheng, H.-M.; Koratkar, N. Superiority of Graphene over Polymer Coatings for Prevention of Microbially Induced Corrosion. Sci. Rep. 2015, 5, 13858. (3) Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: Corrosion-Inhibiting Coating. ACS Nano 2012, 6, 1102−1108. (4) Qi, J. S.; Huang, J. Y.; Feng, J.; Shi, D. N.; Li, J. The Possibility of Chemically Inert, Graphene-Based All-Carbon Electronic Devices with 0.8 eV Gap. ACS Nano 2011, 5, 3475−3482. (5) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (6) Ambrosi, A.; Chua, C. K.; Latiff, N. M.; Loo, A. H.; Wong, C. H. A.; Eng, A. Y. S.; Bonanni, A.; Pumera, M. Graphene and Its Electrochemistry − an Update. Chem. Soc. Rev. 2016, 45, 2458−2493. (7) Eng, A. Y. S.; Chua, C. K.; Pumera, M. Refinements to the Structure of Graphite Oxide: Absolute Quantification of Functional Groups via Selective Labelling. Nanoscale 2015, 7, 20256−20266. (8) Dimiev, A. M.; Alemany, L. B.; Tour, J. M. Graphene Oxide. Origin of Acidity, Its Instability in Water, and a New Dynamic Structural Model. ACS Nano 2013, 7, 576−588. (9) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene Oxide as a Chemically Tunable Platform for Optical Applications. Nat. Chem. 2010, 2, 1015−1024. (10) Tang, Q.; Zhou, Z.; Chen, Z. Graphene-Related Nanomaterials: Tuning Properties by Functionalization. Nanoscale 2013, 5, 4541− 4583. (11) Taniguchi, T.; Yokoi, H.; Nagamine, M.; Tateishi, H.; Funatsu, A.; Hatakeyama, K.; Ogata, C.; Ichida, M.; Ando, H.; Koinuma, M.; Matsumoto, Y. Correlated Optical and Magnetic Properties in Photoreduced Graphene Oxide. J. Phys. Chem. C 2014, 118, 28258− 28265. (12) Wang, S.; Cole, I. S.; Zhao, D.; Li, Q. The Dual Roles of Functional Groups in the Photoluminescence of Graphene Quantum Dots. Nanoscale 2016, 8, 7449−7458. (13) Pan, N.; Guan, D.; Yang, Y.; Huang, Z.; Wang, R.; Jin, Y.; Xia, C. A Rapid Low-Temperature Synthetic Method Leading to Large-Scale Carboxyl Graphene. Chem. Eng. J. 2014, 236, 471−479. (14) Song, M.; Xu, J.; Wu, C. The Effect of Surface Functionalization on the Immobilization of Gold Nanoparticles on Graphene Sheets. J. Nanotechnol. 2012, 2012, 329318. (15) Bjerglund, E.; Kongsfelt, M.; Shimizu, K.; Jensen, B. B. E.; Koefoed, L.; Ceccato, M.; Skrydstrup, T.; Pedersen, S. U.; Daasbjerg,

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07746. High resolution carbon 1s X-ray photoelectron spectra of carboxylated-graphenes, atomic compositions of carboxylated-graphenes based on elemental combustion analysis, energy-dispersive X-ray spectroscopy mapping and elemental compositions, cyclic voltammograms for ferrocyanide at carboxylated-graphene electrodes, photos of carboxylated-graphene dispersions in N,N-dimethylformamide, Tauc plots and excitation−emission maps of 1796


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Graphene Nanoflake Films with Sharp Edge Planes. Adv. Funct. Mater. 2008, 18, 3506−3514. (38) Tauc, J.; Grigorovici, R.; Vancu, A. Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi B 1966, 15, 627−637. (39) Mathkar, A.; Tozier, D.; Cox, P.; Ong, P.; Galande, C.; Balakrishnan, K.; Reddy, A. L. M.; Ajayan, P. M. Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide. J. Phys. Chem. Lett. 2012, 3, 986−991. (40) Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−435. (41) Rourke, J. P.; Pandey, P. A.; Moore, J. J.; Bates, M.; Kinloch, I. A.; Young, R. J.; Wilson, N. R. The Real Graphene Oxide Revealed: Stripping the Oxidative Debris from the Graphene-Like Sheets. Angew. Chem., Int. Ed. 2011, 50, 3173−3177. (42) Naumov, A.; Grote, F.; Overgaard, M.; Roth, A.; Halbig, C. E.; Nørgaard, K.; Guldi, D. M.; Eigler, S. Graphene Oxide: A One- Versus Two-Component Material. J. Am. Chem. Soc. 2016, 138, 11445− 11448. (43) Eng, A. Y. S.; Chua, C. K.; Pumera, M. Intrinsic Electrochemical Performance and Precise Control of Surface Porosity of GrapheneModified Electrodes Using the Drop-Casting Technique. Electrochem. Commun. 2015, 59, 86−90.

K. Controlled Electrochemical Carboxylation of Graphene to Create a Versatile Chemical Platform for Further Functionalization. Langmuir 2014, 30, 6622−6628. (16) Bonanni, A.; Chua, C. K.; Pumera, M. Rational Design of Carboxyl Groups Perpendicularly Attached to a Graphene Sheet: a Platform for Enhanced Biosensing Applications. Chem. - Eur. J. 2014, 20, 217−222. (17) Kolbe, H. Ueber Synthese der Salicylsäure. Justus Liebigs Ann. Chem. 1860, 113, 125−127. (18) Schmitt, R. Beitrag zur Kenntniss der Kolbe’schen Salicylsäure Synthese. J. Prakt. Chem. 1885, 31, 397−411. (19) Lindsey, A. S.; Jeskey, H. The Kolbe-Schmitt Reaction. Chem. Rev. 1957, 57, 583−620. (20) Barroso-Bujans, F.; Alegría, Á .; Colmenero, J. Kinetic Study of the Graphite Oxide Reduction: Combined Structural and Gravimetric Experiments Under Isothermal and Nonisothermal Conditions. J. Phys. Chem. C 2010, 114, 21645−21651. (21) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806−4814. (22) Eigler, S.; Dotzer, C.; Hirsch, A.; Enzelberger, M.; Müller, P. Formation and Decomposition of CO2 Intercalated Graphene Oxide. Chem. Mater. 2012, 24, 1276−1282. (23) Sofer, Z.; Šimek, P.; Pumera, M. Complex Organic Molecules Are Released During Thermal Reduction of Graphite Oxides. Phys. Chem. Chem. Phys. 2013, 15, 9257−9264. (24) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural Evolution During the Reduction of Chemically Derived Graphene Oxide. Nat. Chem. 2010, 2, 581−587. (25) Sofer, Z.; Jankovský, O.; Šimek, P.; Sedmidubský, D.; Šturala, J.; Kosina, J.; Mikšová, R.; Macková, A.; Mikulics, M.; Pumera, M. Insight into the Mechanism of the Thermal Reduction of Graphite Oxide: Deuterium-Labeled Graphite Oxide Is the Key. ACS Nano 2015, 9, 5478−5485. (26) Staudenmaier, L. Verfahren zur Darstellung der Graphitsäure. Ber. Dtsch. Chem. Ges. 1898, 31, 1481−1487. (27) Hofmann, U.; Konig, E. Untersuchungen über Graphitoxyd. Z. Anorg. Allg. Chem. 1937, 234, 311−336. (28) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (29) Wang, Z. Comprehensive Organic Name Reactions and Reagents; John Wiley & Sons: Hoboken, NJ, 2010; pp 1664−1667. (30) Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Vyvyan, J. R. Introduction to Spectroscopy, 4th ed.; Brooks/Cole: Belmont, CA, 2009; pp 15−104. (31) Coates, J. Interpretation of Infrared Spectra, a Practical Approach. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley: Chichester, NY, 2000; pp 10815−10837. (32) Szabo, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740−2749. (33) Eng, A. Y. S.; Poh, H. L.; Luxa, J.; Sofer, Z.; Pumera, M. Potassium Assisted Reduction and Doping of Graphene Oxides: Towards Faster Electron Transfer Kinetics. RSC Adv. 2013, 3, 10900− 10908. (34) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud’homme, R. K.; Aksay, I. A.; Car, R. Raman Spectra of Graphite Oxide and Functionalized Graphene Sheets. Nano Lett. 2008, 8, 36−41. (35) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Spectroscopy. Nano Lett. 2010, 10, 751−758. (36) Brownson, D. A. C.; Kampouris, D. K.; Banks, C. E. Graphene Electrochemistry: Fundamental Concepts Through to Prominent Applications. Chem. Soc. Rev. 2012, 41, 6944−6976. (37) Shang, N. G.; Papakonstantinou, P.; McMullan, M.; Chu, M.; Stamboulis, A.; Potenza, A.; Dhesi, S. S.; Marchetto, H. Catalyst-Free Efficient Growth, Orientation and Biosensing Properties of Multilayer 1797


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