DOI: 10.1002/chem.201406168
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& Graphene
Hydroboration of Graphene Oxide: Towards Stoichiometric Graphol and Hydoroxygraphane Hwee Ling Poh,[a] Zdeneˇk Sofer,[b] Petr Sˇimek,[b] Ivo Tomandl,[c] and Martin Pumera*[a]
H2O2 or CF3COOH treatment as source of OH or H, we can obtain highly hydroxylated compounds of precisely defined composition with a general formula (C1O0.78H0.75)n, which we named graphol and highly hydroxylated graphane (C1(OH)0.51H0.14)n, respectively. These highly functionalized materials with an accurately defined composition are highly important for the field of graphene derivatives. The enhanced electrochemical performance towards important biomarkers as well as hydrogen evolution reaction is demonstrated.
Abstract: Covalently functionalized graphene materials with well-defined stoichiometric composition are of a very high importance in the research of 2D carbon material family due to their well-defined properties. Unfortunately, most of the contemporary graphene-functionalized materials do not have this kind of defined composition and, usually, the amount of heteroatoms bonded to graphene framework is in the range of 1–10 at. %. Herein, we show that by a wellestablished hydroboration reaction chain, which introduces BH2 groups into the graphene oxide structure, followed by
Introduction
Herein, we use the well-established classical synthetic chemistry reaction of hydroboration with subsequent oxidation for fabrication of stoichiometric graphol (C1O1H1)n and with acidic hydrolysis for hydroxylated hydrogenated graphene (hydroxygraphane). Hydroboration is an addition reaction on a C= C bond that results in a HCCBH2 moiety (Scheme 1). The CBH2 bond can consequently be transferred to a COH or C H bond by treatment with H2O2 or with a proton donor (i.e., CF3COOH). The reactivity of borane is very high so other oxygen functional groups present on graphene oxide surface also undergo reduction to an OH group together with partial hydrogenation. For the reaction schemes on various functional groups see Scheme 1. We were able to perform these reactions in very high yields and we obtained highly hydroxylated and hydrogenated graphene materials with the C/H/O ratio close to the stoichiometric graphol and hydroxygraphane. Their characterization was performed by combustible elemental analysis, X-ray photoelectron spectroscopy (XPS), prompt gamma-ray activation analysis (PGAA), and combined scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The electrochemical properties of the resulting materials were also investigated.
Graphene shows very interesting material, physical, and chemical properties and it has been extensively studied[1–21] because it is useful building block of nanoarchitectonic devices.[4, 5] To tune the properties of graphene for useful applications, covalent derivatization of graphene with heteroatom functionalities is often used. These heteroatoms include sulfur,[6] nitrogen,[7] phosphorus,[8] halogen,[9, 10] or hydrogen.[11] Unfortunately, most of the methods result in non-stoichiometric products, (CXz)n, in which z is significantly smaller than 1 and typically in 0.01 < z < 0.1 region,[12] even though some exceptions exist, for example, (C1Hz)n and (C1Fz)n, in which z = 0.7–1.0.[13–15] Other examples include fluorographane with the formula (C1HxF1x)n and thiofluorographene.[16, 17] Thus, only a very limited number of works show a stoichiometric graphene derivative. Stoichiometric derivatives of graphene are of high importance as they are well defined with precisely defined material, physical, and chemical properties. Well-established synthetic chemistry reactions on graphene were proposed to aid this aim. [a] H. L. Poh, Prof. M. Pumera Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University, Singapore 637371 (Singapore) E-mail: [email protected] [b] Prof. Z. Sofer, P. Sˇimek
Results and Discussion The synthesis of highly hydroxylated graphene (dubbed graphol) and hydroxygraphane has been successfully realized through the reaction of graphene with borane (BH3) in the form of a borane-THF complex. The synthesized product obtained from the reaction is an alkylborane compound. It was then subjected to further reactions with trifluoroacetic acid (CF3COOH) and hydrogen peroxide/sodium hydroxide mixture (H2O2/NaOH) to prepare hydroxygraphane and highly hydroxy-
Department of Inorganic Chemistry University of Chemistry and Technology Prague Technick 5, 166 28 Prague 6 (Czech Republic) [c] Dr. I. Tomandl Nuclear Physics Institute of the ASCR Rˇezˇ 130, 250 68 Rˇezˇ (Czech Republic) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406168. Chem. Eur. J. 2015, 21, 1 – 8
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Scheme 2. Hydroboration of graphene oxide C=C bond with consequent hydroxylation or hydrogenation.
the results of elemental analysis one can calculate the composition of HOBD[BH3/H2O2/NaOH] and obtain (C1O0.78H0.75)n (which can be written as (C1O0.03(OH)0.75)n given the fact that hydroboration reactions used (Scheme 1) lead to reduction of oxygen-containing groups to OH and addition of OH on the graphene backbone), which is close to the stoichiometric composition of hydroxylated graphene (graphol) (C1O1H1)n. Due to the hydrogenation mechanism, we expect the vast majority of the functional groups on its surface to be hydroxyls. The hydroboration reaction consequently followed by a substitution of the BH2 group with a proton from CF3COOH led to a composition (C1H0.65O0.51)n. Given the fact that hydroboration with consequent treatment with CF3COOH leads to reduction of oxygen containing groups to OH and addition of H onto carbon backbone, we can write the final formula as (C1(OH)0.51H0.14)n, which is hydroxylated partially hydrogenated graphene. We it term hydroxygraphane. Boron concentration was highest for HOBD[BH3/CF3COOH] at 1.82 wt. % followed by 1.61 wt. % for HOBD[BH3] and, finally, by 1.16 wt. % for HO BD[BH3/H2O2/NaOH]. The results of the boron concentration measurement using PGAA are shown in Figure 1. It should be mentioned that the final material is not completely stoichiometric as well as there may be remaining carboxylic groups from graphene oxide that
Scheme 1. Hydroboration of graphene oxide with consequent hydroxylation or hydrogenation for various functional groups.
lated graphene, respectively (Scheme 2). This work displays the use of a well-known reaction mechanism to introduce hydroxyl groups (OH) and hydrogen (H) into alkylborane. The presented materials to be compared are the alkylborane derivative of graphene (HOBD[BH3]), graphane (HOBD[BH3/CF3COOH]), graphol (HOBD[BH3/H2O2/NaOH]), and thermally reduced graphite oxide untreated with borane (HO-TRG). A full spectrum of characterization procedures of the obtained materials was acquired, including scanning electron microscopy (SEM), Raman spectroscopy, prompt gamma-ray activation analysis (PGAA), X-ray photoelectron spectroscopy (XPS), and combustion elemental analysis (EA). The electrochemical behavior of these materials was investigated with cyclic voltammetry (CV) to observe their characteristics in both the phosphate buffer and ferro/ferricyanide solutions.
Table 1. Elemental composition of hydroborated graphene, graphol, and hydrogenated graphene based on combustible elemental analysis.[a]
Elemental analysis Combustible elemental analysis in combination with prompt gamma-ray activation analysis (PGAA) provided information on the elemental composition of the materials, as shown in Table 1. The amount of carbon was highest for HOBD[BH3] with 56.6 wt. % followed by HOBD[BH3/CF3COOH] at 56.5 wt. %, and, lastly, by HOBD[BH3/H2O2/NaOH] with 48.9 wt. %. From &
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Sample
N
C
H
O
B
HOBD[BH3] HOBD[BH3/CF3COOH] HOBD[BH3/H2O2/NaO]
0.0 0.0 0.0
56.6 56.5 48.9
2.8 3.1 3.0
38.6 38.0 46.9
1.6 1.8 1.2
[a] All values are in wt. %.
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Full Paper Raman spectroscopy Raman spectroscopy was then performed to gain further insight into the surface characteristics of these materials. Obtained Raman spectra (Figure 3 A) showed distinctive peaks at approximately 1350 and about 1560 cm1, which correspond to the sp3 hybridization defects (D-band) and the pristine sp2 lattice (G-band) structure, respectively. The intensities of the D and G bands can provide insight into the density of defects in
Figure 1. Prompt gamma-ray analysis of hydroborated graphene, graphol, and hydroxygraphane.
are not reducible; these are present in the graphene oxide in typical amount of 1–2 % of all oxygen-containing groups.
Electron microscopy Scanning electron microscopy of the different materials was performed to observe their morphological and topographical details. SEM images were obtained for the magnifications of 370 , 6000 , and 50 000 . The images of the materials are shown in Figure 2, in which Figure 2 A shows exfoliated graphene sheets of HOBD[BH3]. Further treatment with CF3COOH (Figure 2 B) resulted in the material displaying a higher degree of exfoliation. However, exposition to HOBD[BH3/H2O2/NaOH] gives a more wrinkled texture, in which smooth and straight graphene sheets were not observed (Figure 2 C). The control material HO-TRG appears to be completely exfoliated to single or few layers of graphene sheets (Figure 2 D). Overall, the materials showed no significant morphological differences between each of them and their features are typical for graphene-based materials.
Figure 3. A) Raman spectra, and B) D/G ratios of the studied materials.
the graphene lattice through calculation of the D/G ratios. The obtained D/G ratios were 1.20 for HOBD[BH3], 1.16 for HO BD[BH3/CF3COOH], 1.22 for HO BD[BH3/H2O2/NaOH], and 1.09 for HO-TRG (Figure 3 B). It can be clearly observed that HO-TRG possessed the lowest amount of defects in its structure, whereas the treated materials have similar amount of defects. Using the intensities of the D and G bands, the average crystallite size (La) of the materials can be calculated with the following Equation:[18] Figure 2. SEM images of: A) hydroborated graphene, B) graphane, C) graphol, and D) untreated thermally reduced graphene. Chem. Eur. J. 2015, 21, 1 – 8
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Full Paper 4 La ¼ 2:4 1010 llaser IG =ID
structure. It is followed by HOBD[BH3/CF3COOH], HOBD[BH3/ H2O2/NaOH], and, finally, by HO-TRG. The peak of pure boron occurs at approximately 188 eV. The appearance of the B1s peak at 193 eV for all samples indicates that boron atoms are incorporated to the graphene structure in a form of boric acid esters and like boric acid, a product of the ester hydrolysis.[21] The relative amount (at. %) of total counts for B1s was computed as shown in the bar graph in Figure 4 D. It was observed that HOBD[BH3] has the highest concentration of B1s (3.69 %), followed by HOBD[BH3/ CF3COOH] at 2.54 %, and, lastly, by 1.15 % for HOBD[BH3/H2O2/ NaOH]. This observed trend can be explained by the reaction scheme shown earlier in which the treatment of graphite oxides only with BH3 will produce graphene sheets with boron functionalities incorporated into the structure. Hence, HO BD[BH3] will possess the highest B1s content compared with other materials. Further reactions with CF3COOH will result in a removal of boron functionalities from the graphene sheets and in the addition of hydrogen atoms into the respective positions. This reaction results in the phenomenon in which HO BD[BH3/CF3COOH] was observed to contain lower B1s content compared with HOBD[BH3]. However, further treatment with H2O2/NaOH in a hydroboration reaction will result in a more effective substitution reaction of the alkylborane as H2O2 is a smaller molecule and more effective substituent. Therefore, the substitution yield is higher when more boron atoms are removed and substituted by hydroxyl groups to give graphol. This causes HOBD[BH3/H2O2/NaOH] to have significantly lower content of boron.
in which llaser is the wavelength of the laser (nm) and IG and ID are the intensities of the G and D bands, respectively. The crystallite sizes of the materials were calculated to give 14.0 nm for HOBD[BH3], 14.4 nm for HOBD[BH3/CF3COOH], 13.7 nm for HOBD[BH3/H2O2/NaOH], and 15.4 nm for HO-TRG. Raman spectra of hydroxygraphane are consistent with those obtained for hydrogenated graphenes prepared by different routes.[19, 20] X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was performed to obtain information on the surface chemistry of the materials, such as elemental composition and character of the bonds. The wide range XPS spectra performed from 0 to 1200 eV are shown in Figure 4 A. Elemental composition was obtained with relative sensitivity factors taken into consideration. The C1s peak at 284.5 eV, O1s peak at 534 eV, and B1s peak at 193 eV can be observed in all spectra, except for the untreated control material, HO-TRG, which shows only the C1s and O1s peaks. The degree of oxidation can be investigated through the C/O ratios, which were calculated from the intensities of the C1s and O1s peaks. The relevant values are 3.79 for HOBD[BH3], 3.97 for HO BD[BH3/CF3COOH], 4.97 for HOBD[BH3/H2O2/NaOH] and 18.22 for HO-TRG. The results are tabulated in Figure 4 B. It can be concluded that HOBD[BH3] underwent oxidation to the greatest degree as it possesses the lowest C/O ratio and, hence, it contains the largest amount of oxygen functionalities in its
Infrared spectroscopy Chemical composition of the samples was further investigated by FTIR (Figure 5). Spectra of HOBD[BH3] and HOBD[BH3/ CF3COOH] are almost identical, nevertheless, a few significant differences were observed. The most important feature is the increase of intensity for the CH vibration bands located at 2950 and 2870 cm1, respectively. In the case of HOBD[BH3/ H2O2/NaOH] this double band is very weak but, on the other hand, a broad band at 3350 cm1 originating from hydroxyl groups appears. It clearly correlates with the reaction mechanism purposed for various functional groups. Background corrected spectra are featured as Figure S1 (in the Supporting Information). The weak band of the C=O vibration at 1700 cm1 is not visible in the spectrum of HOBD[BH3/H2O2/NaOH] but is present in HOBD[BH3] and HOBD[BH3/CF3COOH]. This indicates an incomplete reduction of the C=O groups within graphene oxide with BH3 and its subsequent decomposition/hydrogenation with trifluoroacetic acid. The skeletal vibration of the C=C bond at 1560 cm1 has the strongest intensity in the case of HOBD[BH3/H2O2/NaOH]. The intensive CO stretching vibration of the COOH group in HOBD[BH3] and HOBD[BH3/CF3COOH] is located at 1420 cm1. This band is much weaker in the case of HOBD[BH3/H2O2/NaOH], which indicates a higher degree of reduction. The broad band originating from the CO stretching
Figure 4. A) Wide-scan XPS spectra, B) C/O ratios calculated from the survey XPS spectra, C) HR-XPS spectra of B1 s, and D) percentage (at. %) of the residual boron calculated from XPS spectra of the functionalized materials.
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Figure 5. FTIR spectra of functionalized graphenes. Top: wide spectra; bottom: enlarged part from 2500 to 4000 cm1. Figure 6. Electrochemical performance. A) Cyclic voltammetry of ascorbic acid (10 mm) in PBS buffer (50 mm, pH 7), and B) linear-sweep voltammetry in 0.5 m H2SO4 showing hydrogen evolution, at various functionalized graphene surfaces.
vibration of the hydroxyl group appears at 1040 cm1 and it is most intense for HOBD[BH3/H2O2/NaOH]. The CO stretching band of HOBD[BH3] and HOBD[BH3/CF3COOH] is more localized and shifted towards lower wavenumbers. These results are in very good agreement with those obtained by using combustible elemental analysis and XPS.
boration reaction allowed us to shift the graphene composition towards hydroxygraphene or hydroxygraphane. It is clear that hydroboration reaction, which adds on a C=C group and further reduces oxygen functionalities, can serve as a starting point for synthesis of highly functionalized graphenes with a well-defined stoichiometry composition. We have demonstrated that such highly hydroxylated graphenes and graphanes show excellent electrocatalytic properties towards biomarkers. Such precisely defined highly functionalized materials are of paramount importance for practical applications of graphene.
Electrochemistry We have tested the hydroxygraphene (graphol) and hydroxygraphane for their utility for the electrochemical detection of biomarkers as well as for hydrogen evolution reaction. Figure 6 A shows that graphol shows the most favorable electrocatalytic properties towards the oxidation of ascorbic acid, which is an important biomarker. This is due to the presence of hydroxyl groups in its structure. Figure 6 B shows that hydroxygraphane and then graphol demonstrate catalytic properties towards hydrogen evolution reaction, whereas graphene does not show any significant electrocatalysis.
Experimental Section Materials High purity microcrystalline graphite (2–15 mm, 99.9995 %) was obtained from Alfa Aesar, Germany. Sulfuric acid (98 %), nitric acid (68 %), potassium chlorate (> 99 %), sodium hydroxide (> 98 %), hydrogen peroxide (30 %), and hydrochloric acid (37 %) were obtained from Penta (Czech Republic). The borane-THF complex (1 m) and trifluoroacetic acid (anhydrous) were obtained from Sigma-Aldrich, Czech Republic. Tetrahydrofuran (> 99.8 %, THF) was obtained from Lach-Ner, Czech Republic and was dried prior to use by distillation from Na/benzophenone under argon atmosphere. Argon of 99.996 % purity was obtained from SIAD, Czech Republic.
Conclusion We have prepared highly hydroxylated compound of the composition of (C1O0.78H0.75)n through hydroboration of graphene oxide consequently followed by treatment with H2O2. In a similar manner, we prepared highly hydroxylated graphane with the composition of C1H0.65O0.51 (C1(OH)0.51H0.14)n by using CF3COOH as a source of protons. Different quenching of hydroChem. Eur. J. 2015, 21, 1 – 8
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Full Paper Potassium ferrocyanide, N,N-dimethylformamide (DMF), potassium chloride, potassium phosphate dibasic, sodium phosphate monobasic, sodium chloride, and sodium phosphite dibasic pentahydrate were obtained from Sigma–Aldrich, Singapore. Deionized water was used in the preparation of the electrolytes used in all the electrochemical measurements. Platinum auxiliary electrode (Pt), glassy carbon working electrode (GC) and Ag/AgCl reference electrode were purchased from Autolab, The Netherlands.
pH7.4 deoxygenated phosphate buffer solution (PBS) and 10 mm ferro/ferricyanide dissolved in phosphate buffer solution. All measurements were performed for two consecutive scans. The scan rate used was 100 mV s1 and three measurements were carried out for each material and also for each electrolyte. Deoxygenation of the 50 mm pH 7.4 PBS was performed by bubbling nitrogen gas into the PBS for 15 min before each use. Cyclic voltammetry was carried out at scan rate of 100 mV s1.
Methods
Synthetic procedures
JEOL 7600F field-emission scanning electron microscope, Japan was used to obtain SEM images of the samples that were affixed onto the sample stub with a conductive carbon tape. A Phoibos 100 spectrometer and a monochromatic 12.53 kV Mg X-ray radiation source (SPECS, Germany) was used to acquire the XPS and HRXPS measurements. An aluminium sample holder and a conductive sticky carbon tape were used to attach the XPS samples. A homogeneous and uniform layer of the material was attached onto the tape before performing an XPS measurement. A CCD detector and a Confocal micro-Raman LabRam HR instrument (Horiba Scientific) in backscattering geometry was used for Raman spectroscopic measurements. Silicon wafer was utilized for the standard for calibration at 0 and 520 cm1 to give a peak position resolution of less than 1 cm1. A 100 objective lens, a 514.5 nm Ar laser, and an Olympus optical microscope were used for all the measurements. All materials were well-compacted prior to any measurement.
Synthesis of graphite oxide: 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 8C, 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 8C, potassium chlorate (55 g) was slowly added to the mixture to avoid a sudden increase in temperature and a consequent formation of explosive chlorine dioxide gas. Upon the complete dissolution of the potassium chlorate, the reaction flask was loosely capped to allow an escape of the evolved gas and the mixture was continuously vigorously stirred for 96 h at room temperature. When the reaction ended, 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. The graphite oxide slurry was then dried in a vacuum oven at 50 8C for 48 h before use.
Combustible elemental analysis (CHNS-O) was performed using a PE 2400 Series II CHNS/O Analyzer (PerkinElmer, USA). The instrument was used in a CHN operating mode (the most robust and interference-free mode) to convert the sample elements to simple gases (CO2, H2O, and N2). The PE 2400 analyzer automatically performed combustion, reduction, homogenization of product gases, separation and detection. An MX5 microbalance (Mettler Toledo) was used for precise weighing of the samples (1.5–2.5 mg per single sample analysis). By using this procedure, the accuracy of CHN determination is better than 0.30 % abs. Internal calibration was performed using an N-fenyl urea.
Synthesis of HO-BD[BH3]: Graphite oxide (500 mg) was dispersed in anhydrous THF (500 mL) by ultrasonication (400 W, 60 min). 1 m BH3 solution in THF (60 mL) was added. The reaction mixture was heated at reflux under argon atmosphere for 24 h. Reduced graphene was separated by a suction filtration under argon atmosphere and repeatedly washed with dry THF. Then it was dried in a vacuum oven for 48 h at 50 8C. Synthesis of HO-BD[BH3/CF3COOH]: Graphite oxide (500 mg) was dispersed in anhydrous THF (500 mL) by ultrasonication (400 W, 60 min). 1 m BH3 solution in THF (60 mL) was added. The reaction mixture was heated at reflux under an argon atmosphere for 24 h. Anhydrous CF3COOH was added and heated at reflux for 3 h under an argon atmosphere. Hydrogenated graphene was separated by a suction filtration, repeatedly washed with THF and water. The product was than dried in a vacuum oven (50 8C, 48 h).
Prompt gamma-ray activation analysis (PGAA) was used to determine boron content in the graphene samples. These PGAA measurements were performed in the PGAA facility, which is installed at a LWR-15 research reactor. Samples were irradiated in a thermal neutron beam with flux of approximately 3 106 n cm2 s1. The gamma spectra were taken with the well-shielded 25 % HPGe detector.
Synthesis of HO-BD[BH3/H2O2/NaOH]: Graphite oxide (500 mg) was dispersed in 500 mL of anhydrous THF by ultrasonication (400 W, 60 min). 1 m BH3 solution (60 mL) in THF was added. The reaction mixture was heated at reflux under an argon atmosphere for 24 h. A mixture of 3 m NaOH/30 % H2O2 (1:1 by volume) was slowly added to the reaction mixture. It was subsequently heated at reflux for 3 h and the formed hydroxylated graphene was separated by a suction filtration and repeatedly washed with THF and water. The product was than dried in a vacuum oven (50 8C, 48 h).
Electrochemical voltammetric measurements were performed by using a microAutolab Type III electrochemical analyzer (Eco Chemie, The Netherlands). A NOVA 1.7 software (Eco Chemie) was used to control the electrochemical analyzer. The Nova 1.7 software was published by Metrohm Autolab B. V. to support of Metrohm Autolab’s potentiostat/galvanostat equipment. For the measurement of cyclic voltammetry the glassy carbon electrodes were renewed by polishing with alumina suspension to clean the electrode surface and then washed and wiped dry before each use. The materials were dispersed in DMF to obtain a 1.0 mg mL1 suspension. The suspension was then sonicated for 5 min at room temperatures prior to each use. A cleaned GC electrode was then modified by drip coating with a 1 mL aliquot of the suspension and left to dry under a lamp to obtain a layer of randomly dispersed material. The modified GC electrodes, Pt counter electrode, and Ag/AgCl reference electrode were then placed into an electrochemical cell that contained the electrolyte. The CV scans were then executed. The electrolytes used were 50 mm
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Synthesis of HO-TRG: Thermally reduced graphene was obtained by thermal exfoliation/reduction of graphite oxide prepared by the Hofmann method (HO-GO) at high temperature in a nitrogen atmosphere. Thermal exfoliation/reduction of graphite oxide was performed at 1000 8C and under pressure of 100 kPa. Graphite oxide (0.1 g) was placed in a porous quartz glass capsule connected to magnetic manipulator inside a vacuum tight tube furnace with a controlled atmosphere. The application of magnetic manipulator allowed us to create temperature gradient over 1000 8C min1. The tube was flushed with nitrogen and repeatedly
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Full Paper evacuated to remove any traces of oxygen. Then the reactor was repeatedly flushed with hydrogen and evacuated and the sample was then quickly inserted by a magnetic manipulator to a preheated segment of the furnace and held in there for 12 min. The flow of hydrogen during the exfoliation procedure was 1000 sccm to remove any evolved byproducts.
[6] a) Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 2012, 6, 205; b) H. L. Poh, P. Simek, Z. Sofer, M. Pumera, ACS Nano 2013, 7, 5262. [7] H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang, J. W. Choi, Nano Lett. 2011, 11, 2472. [8] H. L. Poh, Z. Sofer, M. Novacek, M. Pumera, Chem. Eur. J. 2014, 20, 4284. [9] F. Karlicky´, K. K. R. Datta, M. Otyepka, R. Zboil, ACS Nano 2013, 7, 6434. [10] H. L. Poh, P. Simek, Z. Sofer, M. Pumera, Chem. Eur. J. 2013, 19, 2655. [11] D. C. Elias, R. R. Nair, T. M. G. Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, Science 2009, 323, 610. [12] M. J. Pumera, J. Mater. Chem. C 2014, 2, 6454. [13] A. Y. S. Eng, H. L. Poh, F. Sanek, M. Marysko, S. Matejkova, Z. Sofer, M. Pumera, ACS Nano 2013, 7, 5930. [14] M. Pumera, C. H. A. Wong, Chem. Soc. Rev. 2013, 42, 5987. [15] R. Zborˇil, F. Karlicky, A. B. Bourlinos, T. A. Steriotis, A. K. Stubos, V. Georgakilas, V. Safrov, D. Janck, C. Trapalis, M. Otyepka, Small 2010, 6, 2885. [16] Z. Sofer, P. Simek, V. Mazanek, F. Sembera, Z. Janousek, M. Pumera, Chem. Commun. 2015, 51, 5633. [17] V. Urbanov, K. Hol, A. Cpe, K. B. Bourlinos, A. Ambrosi, A. H. Loo, M. Pumera, F. Karlicky´, M. Otyepka, R. Zboril, Adv. Mater. 2015, 27, 2305. [18] L. G. CanÅado, K. Takai, T. Enoki, M. Endo, Y. A. Kim, H. Mizusaki, A. Jorio, L. N. Coelho, R. Magalhaes-Paniago, M. A. Pimenta, Appl. Phys. Lett. 2006, 88, 163106. [19] K. S. Subrahmanyam, P. Kumar, U. Maitra, A. Govindaraj, K. P. S. S. Hembram, U. V. Waghmare, C. N. R. Rao, Proc. Natl. Acad. Sci. USA 2011, 108, 2674. [20] Z. Yang, Y. Sun, L. B. Alemany, T. N. Narayanan, W. E. Billups, J. Am. Chem. Soc. 2012, 134, 18689. [21] E. Pourazadi, E. Haque, W. Zhang, A. T. Harrisa, A. I. Minett, Chem. Commun. 2013, 49, 11068.
Acknowledgements Z.S. and P.S. were supported by Specific University Research (MSMT No 20/2015) and the Czech Science Foundation (GACR No. 15-09001S). M.P. and H.L.P. acknowledge a Tier 2 grant from the Ministry of Education, Singapore (MOE2013-T2-1-056; ARC 35/13). Measurements of PGAA were carried out at the CANAM infrastructure of the NPI ASCR Rˇezˇ supported by the Ministry of Education, Youth and Sports (grant project No. LM2011019). Keywords: boranes · electrochemistry hydroboration · synthetic methods
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Received: November 21, 2014 Revised: March 10, 2015 Published online on && &&, 2015
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FULL PAPER & Graphene
Stand out from the crowd: By using a well-established hydroboration reaction chain it is possible to obtain highly hydroxylated compounds of precisely defined composition with a general formula (C1O0.78H0.75)n, named graphol and highly hydroxylated graphane (C1(OH)0.51H0.14)n (see figure). Compared with graphene, the compounds demonstrate enhanced electrochemical performance.
H. L. Poh, Z. Sofer, P. Sˇimek, I. Tomandl, M. Pumera* && – && Hydroboration of Graphene Oxide: Towards Stoichiometric Graphol and Hydoroxygraphane
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& Graphene
Hydroboration of Graphene Oxide: Towards Stoichiometric Graphol and Hydoroxygraphane Hwee Ling Poh,[a] Zdeneˇk Sofer,[b] Petr Sˇimek,[b] Ivo Tomandl,[c] and Martin Pumera*[a]
H2O2 or CF3COOH treatment as source of OH or H, we can obtain highly hydroxylated compounds of precisely defined composition with a general formula (C1O0.78H0.75)n, which we named graphol and highly hydroxylated graphane (C1(OH)0.51H0.14)n, respectively. These highly functionalized materials with an accurately defined composition are highly important for the field of graphene derivatives. The enhanced electrochemical performance towards important biomarkers as well as hydrogen evolution reaction is demonstrated.
Abstract: Covalently functionalized graphene materials with well-defined stoichiometric composition are of a very high importance in the research of 2D carbon material family due to their well-defined properties. Unfortunately, most of the contemporary graphene-functionalized materials do not have this kind of defined composition and, usually, the amount of heteroatoms bonded to graphene framework is in the range of 1–10 at. %. Herein, we show that by a wellestablished hydroboration reaction chain, which introduces BH2 groups into the graphene oxide structure, followed by
Introduction
Herein, we use the well-established classical synthetic chemistry reaction of hydroboration with subsequent oxidation for fabrication of stoichiometric graphol (C1O1H1)n and with acidic hydrolysis for hydroxylated hydrogenated graphene (hydroxygraphane). Hydroboration is an addition reaction on a C= C bond that results in a HCCBH2 moiety (Scheme 1). The CBH2 bond can consequently be transferred to a COH or C H bond by treatment with H2O2 or with a proton donor (i.e., CF3COOH). The reactivity of borane is very high so other oxygen functional groups present on graphene oxide surface also undergo reduction to an OH group together with partial hydrogenation. For the reaction schemes on various functional groups see Scheme 1. We were able to perform these reactions in very high yields and we obtained highly hydroxylated and hydrogenated graphene materials with the C/H/O ratio close to the stoichiometric graphol and hydroxygraphane. Their characterization was performed by combustible elemental analysis, X-ray photoelectron spectroscopy (XPS), prompt gamma-ray activation analysis (PGAA), and combined scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The electrochemical properties of the resulting materials were also investigated.
Graphene shows very interesting material, physical, and chemical properties and it has been extensively studied[1–21] because it is useful building block of nanoarchitectonic devices.[4, 5] To tune the properties of graphene for useful applications, covalent derivatization of graphene with heteroatom functionalities is often used. These heteroatoms include sulfur,[6] nitrogen,[7] phosphorus,[8] halogen,[9, 10] or hydrogen.[11] Unfortunately, most of the methods result in non-stoichiometric products, (CXz)n, in which z is significantly smaller than 1 and typically in 0.01 < z < 0.1 region,[12] even though some exceptions exist, for example, (C1Hz)n and (C1Fz)n, in which z = 0.7–1.0.[13–15] Other examples include fluorographane with the formula (C1HxF1x)n and thiofluorographene.[16, 17] Thus, only a very limited number of works show a stoichiometric graphene derivative. Stoichiometric derivatives of graphene are of high importance as they are well defined with precisely defined material, physical, and chemical properties. Well-established synthetic chemistry reactions on graphene were proposed to aid this aim. [a] H. L. Poh, Prof. M. Pumera Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University, Singapore 637371 (Singapore) E-mail: [email protected] [b] Prof. Z. Sofer, P. Sˇimek
Results and Discussion The synthesis of highly hydroxylated graphene (dubbed graphol) and hydroxygraphane has been successfully realized through the reaction of graphene with borane (BH3) in the form of a borane-THF complex. The synthesized product obtained from the reaction is an alkylborane compound. It was then subjected to further reactions with trifluoroacetic acid (CF3COOH) and hydrogen peroxide/sodium hydroxide mixture (H2O2/NaOH) to prepare hydroxygraphane and highly hydroxy-
Department of Inorganic Chemistry University of Chemistry and Technology Prague Technick 5, 166 28 Prague 6 (Czech Republic) [c] Dr. I. Tomandl Nuclear Physics Institute of the ASCR Rˇezˇ 130, 250 68 Rˇezˇ (Czech Republic) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201406168. Chem. Eur. J. 2015, 21, 1 – 8
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Scheme 2. Hydroboration of graphene oxide C=C bond with consequent hydroxylation or hydrogenation.
the results of elemental analysis one can calculate the composition of HOBD[BH3/H2O2/NaOH] and obtain (C1O0.78H0.75)n (which can be written as (C1O0.03(OH)0.75)n given the fact that hydroboration reactions used (Scheme 1) lead to reduction of oxygen-containing groups to OH and addition of OH on the graphene backbone), which is close to the stoichiometric composition of hydroxylated graphene (graphol) (C1O1H1)n. Due to the hydrogenation mechanism, we expect the vast majority of the functional groups on its surface to be hydroxyls. The hydroboration reaction consequently followed by a substitution of the BH2 group with a proton from CF3COOH led to a composition (C1H0.65O0.51)n. Given the fact that hydroboration with consequent treatment with CF3COOH leads to reduction of oxygen containing groups to OH and addition of H onto carbon backbone, we can write the final formula as (C1(OH)0.51H0.14)n, which is hydroxylated partially hydrogenated graphene. We it term hydroxygraphane. Boron concentration was highest for HOBD[BH3/CF3COOH] at 1.82 wt. % followed by 1.61 wt. % for HOBD[BH3] and, finally, by 1.16 wt. % for HO BD[BH3/H2O2/NaOH]. The results of the boron concentration measurement using PGAA are shown in Figure 1. It should be mentioned that the final material is not completely stoichiometric as well as there may be remaining carboxylic groups from graphene oxide that
Scheme 1. Hydroboration of graphene oxide with consequent hydroxylation or hydrogenation for various functional groups.
lated graphene, respectively (Scheme 2). This work displays the use of a well-known reaction mechanism to introduce hydroxyl groups (OH) and hydrogen (H) into alkylborane. The presented materials to be compared are the alkylborane derivative of graphene (HOBD[BH3]), graphane (HOBD[BH3/CF3COOH]), graphol (HOBD[BH3/H2O2/NaOH]), and thermally reduced graphite oxide untreated with borane (HO-TRG). A full spectrum of characterization procedures of the obtained materials was acquired, including scanning electron microscopy (SEM), Raman spectroscopy, prompt gamma-ray activation analysis (PGAA), X-ray photoelectron spectroscopy (XPS), and combustion elemental analysis (EA). The electrochemical behavior of these materials was investigated with cyclic voltammetry (CV) to observe their characteristics in both the phosphate buffer and ferro/ferricyanide solutions.
Table 1. Elemental composition of hydroborated graphene, graphol, and hydrogenated graphene based on combustible elemental analysis.[a]
Elemental analysis Combustible elemental analysis in combination with prompt gamma-ray activation analysis (PGAA) provided information on the elemental composition of the materials, as shown in Table 1. The amount of carbon was highest for HOBD[BH3] with 56.6 wt. % followed by HOBD[BH3/CF3COOH] at 56.5 wt. %, and, lastly, by HOBD[BH3/H2O2/NaOH] with 48.9 wt. %. From &
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Sample
N
C
H
O
B
HOBD[BH3] HOBD[BH3/CF3COOH] HOBD[BH3/H2O2/NaO]
0.0 0.0 0.0
56.6 56.5 48.9
2.8 3.1 3.0
38.6 38.0 46.9
1.6 1.8 1.2
[a] All values are in wt. %.
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Full Paper Raman spectroscopy Raman spectroscopy was then performed to gain further insight into the surface characteristics of these materials. Obtained Raman spectra (Figure 3 A) showed distinctive peaks at approximately 1350 and about 1560 cm1, which correspond to the sp3 hybridization defects (D-band) and the pristine sp2 lattice (G-band) structure, respectively. The intensities of the D and G bands can provide insight into the density of defects in
Figure 1. Prompt gamma-ray analysis of hydroborated graphene, graphol, and hydroxygraphane.
are not reducible; these are present in the graphene oxide in typical amount of 1–2 % of all oxygen-containing groups.
Electron microscopy Scanning electron microscopy of the different materials was performed to observe their morphological and topographical details. SEM images were obtained for the magnifications of 370 , 6000 , and 50 000 . The images of the materials are shown in Figure 2, in which Figure 2 A shows exfoliated graphene sheets of HOBD[BH3]. Further treatment with CF3COOH (Figure 2 B) resulted in the material displaying a higher degree of exfoliation. However, exposition to HOBD[BH3/H2O2/NaOH] gives a more wrinkled texture, in which smooth and straight graphene sheets were not observed (Figure 2 C). The control material HO-TRG appears to be completely exfoliated to single or few layers of graphene sheets (Figure 2 D). Overall, the materials showed no significant morphological differences between each of them and their features are typical for graphene-based materials.
Figure 3. A) Raman spectra, and B) D/G ratios of the studied materials.
the graphene lattice through calculation of the D/G ratios. The obtained D/G ratios were 1.20 for HOBD[BH3], 1.16 for HO BD[BH3/CF3COOH], 1.22 for HO BD[BH3/H2O2/NaOH], and 1.09 for HO-TRG (Figure 3 B). It can be clearly observed that HO-TRG possessed the lowest amount of defects in its structure, whereas the treated materials have similar amount of defects. Using the intensities of the D and G bands, the average crystallite size (La) of the materials can be calculated with the following Equation:[18] Figure 2. SEM images of: A) hydroborated graphene, B) graphane, C) graphol, and D) untreated thermally reduced graphene. Chem. Eur. J. 2015, 21, 1 – 8
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Full Paper 4 La ¼ 2:4 1010 llaser IG =ID
structure. It is followed by HOBD[BH3/CF3COOH], HOBD[BH3/ H2O2/NaOH], and, finally, by HO-TRG. The peak of pure boron occurs at approximately 188 eV. The appearance of the B1s peak at 193 eV for all samples indicates that boron atoms are incorporated to the graphene structure in a form of boric acid esters and like boric acid, a product of the ester hydrolysis.[21] The relative amount (at. %) of total counts for B1s was computed as shown in the bar graph in Figure 4 D. It was observed that HOBD[BH3] has the highest concentration of B1s (3.69 %), followed by HOBD[BH3/ CF3COOH] at 2.54 %, and, lastly, by 1.15 % for HOBD[BH3/H2O2/ NaOH]. This observed trend can be explained by the reaction scheme shown earlier in which the treatment of graphite oxides only with BH3 will produce graphene sheets with boron functionalities incorporated into the structure. Hence, HO BD[BH3] will possess the highest B1s content compared with other materials. Further reactions with CF3COOH will result in a removal of boron functionalities from the graphene sheets and in the addition of hydrogen atoms into the respective positions. This reaction results in the phenomenon in which HO BD[BH3/CF3COOH] was observed to contain lower B1s content compared with HOBD[BH3]. However, further treatment with H2O2/NaOH in a hydroboration reaction will result in a more effective substitution reaction of the alkylborane as H2O2 is a smaller molecule and more effective substituent. Therefore, the substitution yield is higher when more boron atoms are removed and substituted by hydroxyl groups to give graphol. This causes HOBD[BH3/H2O2/NaOH] to have significantly lower content of boron.
in which llaser is the wavelength of the laser (nm) and IG and ID are the intensities of the G and D bands, respectively. The crystallite sizes of the materials were calculated to give 14.0 nm for HOBD[BH3], 14.4 nm for HOBD[BH3/CF3COOH], 13.7 nm for HOBD[BH3/H2O2/NaOH], and 15.4 nm for HO-TRG. Raman spectra of hydroxygraphane are consistent with those obtained for hydrogenated graphenes prepared by different routes.[19, 20] X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) was performed to obtain information on the surface chemistry of the materials, such as elemental composition and character of the bonds. The wide range XPS spectra performed from 0 to 1200 eV are shown in Figure 4 A. Elemental composition was obtained with relative sensitivity factors taken into consideration. The C1s peak at 284.5 eV, O1s peak at 534 eV, and B1s peak at 193 eV can be observed in all spectra, except for the untreated control material, HO-TRG, which shows only the C1s and O1s peaks. The degree of oxidation can be investigated through the C/O ratios, which were calculated from the intensities of the C1s and O1s peaks. The relevant values are 3.79 for HOBD[BH3], 3.97 for HO BD[BH3/CF3COOH], 4.97 for HOBD[BH3/H2O2/NaOH] and 18.22 for HO-TRG. The results are tabulated in Figure 4 B. It can be concluded that HOBD[BH3] underwent oxidation to the greatest degree as it possesses the lowest C/O ratio and, hence, it contains the largest amount of oxygen functionalities in its
Infrared spectroscopy Chemical composition of the samples was further investigated by FTIR (Figure 5). Spectra of HOBD[BH3] and HOBD[BH3/ CF3COOH] are almost identical, nevertheless, a few significant differences were observed. The most important feature is the increase of intensity for the CH vibration bands located at 2950 and 2870 cm1, respectively. In the case of HOBD[BH3/ H2O2/NaOH] this double band is very weak but, on the other hand, a broad band at 3350 cm1 originating from hydroxyl groups appears. It clearly correlates with the reaction mechanism purposed for various functional groups. Background corrected spectra are featured as Figure S1 (in the Supporting Information). The weak band of the C=O vibration at 1700 cm1 is not visible in the spectrum of HOBD[BH3/H2O2/NaOH] but is present in HOBD[BH3] and HOBD[BH3/CF3COOH]. This indicates an incomplete reduction of the C=O groups within graphene oxide with BH3 and its subsequent decomposition/hydrogenation with trifluoroacetic acid. The skeletal vibration of the C=C bond at 1560 cm1 has the strongest intensity in the case of HOBD[BH3/H2O2/NaOH]. The intensive CO stretching vibration of the COOH group in HOBD[BH3] and HOBD[BH3/CF3COOH] is located at 1420 cm1. This band is much weaker in the case of HOBD[BH3/H2O2/NaOH], which indicates a higher degree of reduction. The broad band originating from the CO stretching
Figure 4. A) Wide-scan XPS spectra, B) C/O ratios calculated from the survey XPS spectra, C) HR-XPS spectra of B1 s, and D) percentage (at. %) of the residual boron calculated from XPS spectra of the functionalized materials.
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Figure 5. FTIR spectra of functionalized graphenes. Top: wide spectra; bottom: enlarged part from 2500 to 4000 cm1. Figure 6. Electrochemical performance. A) Cyclic voltammetry of ascorbic acid (10 mm) in PBS buffer (50 mm, pH 7), and B) linear-sweep voltammetry in 0.5 m H2SO4 showing hydrogen evolution, at various functionalized graphene surfaces.
vibration of the hydroxyl group appears at 1040 cm1 and it is most intense for HOBD[BH3/H2O2/NaOH]. The CO stretching band of HOBD[BH3] and HOBD[BH3/CF3COOH] is more localized and shifted towards lower wavenumbers. These results are in very good agreement with those obtained by using combustible elemental analysis and XPS.
boration reaction allowed us to shift the graphene composition towards hydroxygraphene or hydroxygraphane. It is clear that hydroboration reaction, which adds on a C=C group and further reduces oxygen functionalities, can serve as a starting point for synthesis of highly functionalized graphenes with a well-defined stoichiometry composition. We have demonstrated that such highly hydroxylated graphenes and graphanes show excellent electrocatalytic properties towards biomarkers. Such precisely defined highly functionalized materials are of paramount importance for practical applications of graphene.
Electrochemistry We have tested the hydroxygraphene (graphol) and hydroxygraphane for their utility for the electrochemical detection of biomarkers as well as for hydrogen evolution reaction. Figure 6 A shows that graphol shows the most favorable electrocatalytic properties towards the oxidation of ascorbic acid, which is an important biomarker. This is due to the presence of hydroxyl groups in its structure. Figure 6 B shows that hydroxygraphane and then graphol demonstrate catalytic properties towards hydrogen evolution reaction, whereas graphene does not show any significant electrocatalysis.
Experimental Section Materials High purity microcrystalline graphite (2–15 mm, 99.9995 %) was obtained from Alfa Aesar, Germany. Sulfuric acid (98 %), nitric acid (68 %), potassium chlorate (> 99 %), sodium hydroxide (> 98 %), hydrogen peroxide (30 %), and hydrochloric acid (37 %) were obtained from Penta (Czech Republic). The borane-THF complex (1 m) and trifluoroacetic acid (anhydrous) were obtained from Sigma-Aldrich, Czech Republic. Tetrahydrofuran (> 99.8 %, THF) was obtained from Lach-Ner, Czech Republic and was dried prior to use by distillation from Na/benzophenone under argon atmosphere. Argon of 99.996 % purity was obtained from SIAD, Czech Republic.
Conclusion We have prepared highly hydroxylated compound of the composition of (C1O0.78H0.75)n through hydroboration of graphene oxide consequently followed by treatment with H2O2. In a similar manner, we prepared highly hydroxylated graphane with the composition of C1H0.65O0.51 (C1(OH)0.51H0.14)n by using CF3COOH as a source of protons. Different quenching of hydroChem. Eur. J. 2015, 21, 1 – 8
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Full Paper Potassium ferrocyanide, N,N-dimethylformamide (DMF), potassium chloride, potassium phosphate dibasic, sodium phosphate monobasic, sodium chloride, and sodium phosphite dibasic pentahydrate were obtained from Sigma–Aldrich, Singapore. Deionized water was used in the preparation of the electrolytes used in all the electrochemical measurements. Platinum auxiliary electrode (Pt), glassy carbon working electrode (GC) and Ag/AgCl reference electrode were purchased from Autolab, The Netherlands.
pH7.4 deoxygenated phosphate buffer solution (PBS) and 10 mm ferro/ferricyanide dissolved in phosphate buffer solution. All measurements were performed for two consecutive scans. The scan rate used was 100 mV s1 and three measurements were carried out for each material and also for each electrolyte. Deoxygenation of the 50 mm pH 7.4 PBS was performed by bubbling nitrogen gas into the PBS for 15 min before each use. Cyclic voltammetry was carried out at scan rate of 100 mV s1.
Methods
Synthetic procedures
JEOL 7600F field-emission scanning electron microscope, Japan was used to obtain SEM images of the samples that were affixed onto the sample stub with a conductive carbon tape. A Phoibos 100 spectrometer and a monochromatic 12.53 kV Mg X-ray radiation source (SPECS, Germany) was used to acquire the XPS and HRXPS measurements. An aluminium sample holder and a conductive sticky carbon tape were used to attach the XPS samples. A homogeneous and uniform layer of the material was attached onto the tape before performing an XPS measurement. A CCD detector and a Confocal micro-Raman LabRam HR instrument (Horiba Scientific) in backscattering geometry was used for Raman spectroscopic measurements. Silicon wafer was utilized for the standard for calibration at 0 and 520 cm1 to give a peak position resolution of less than 1 cm1. A 100 objective lens, a 514.5 nm Ar laser, and an Olympus optical microscope were used for all the measurements. All materials were well-compacted prior to any measurement.
Synthesis of graphite oxide: 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 8C, 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 8C, potassium chlorate (55 g) was slowly added to the mixture to avoid a sudden increase in temperature and a consequent formation of explosive chlorine dioxide gas. Upon the complete dissolution of the potassium chlorate, the reaction flask was loosely capped to allow an escape of the evolved gas and the mixture was continuously vigorously stirred for 96 h at room temperature. When the reaction ended, 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. The graphite oxide slurry was then dried in a vacuum oven at 50 8C for 48 h before use.
Combustible elemental analysis (CHNS-O) was performed using a PE 2400 Series II CHNS/O Analyzer (PerkinElmer, USA). The instrument was used in a CHN operating mode (the most robust and interference-free mode) to convert the sample elements to simple gases (CO2, H2O, and N2). The PE 2400 analyzer automatically performed combustion, reduction, homogenization of product gases, separation and detection. An MX5 microbalance (Mettler Toledo) was used for precise weighing of the samples (1.5–2.5 mg per single sample analysis). By using this procedure, the accuracy of CHN determination is better than 0.30 % abs. Internal calibration was performed using an N-fenyl urea.
Synthesis of HO-BD[BH3]: Graphite oxide (500 mg) was dispersed in anhydrous THF (500 mL) by ultrasonication (400 W, 60 min). 1 m BH3 solution in THF (60 mL) was added. The reaction mixture was heated at reflux under argon atmosphere for 24 h. Reduced graphene was separated by a suction filtration under argon atmosphere and repeatedly washed with dry THF. Then it was dried in a vacuum oven for 48 h at 50 8C. Synthesis of HO-BD[BH3/CF3COOH]: Graphite oxide (500 mg) was dispersed in anhydrous THF (500 mL) by ultrasonication (400 W, 60 min). 1 m BH3 solution in THF (60 mL) was added. The reaction mixture was heated at reflux under an argon atmosphere for 24 h. Anhydrous CF3COOH was added and heated at reflux for 3 h under an argon atmosphere. Hydrogenated graphene was separated by a suction filtration, repeatedly washed with THF and water. The product was than dried in a vacuum oven (50 8C, 48 h).
Prompt gamma-ray activation analysis (PGAA) was used to determine boron content in the graphene samples. These PGAA measurements were performed in the PGAA facility, which is installed at a LWR-15 research reactor. Samples were irradiated in a thermal neutron beam with flux of approximately 3 106 n cm2 s1. The gamma spectra were taken with the well-shielded 25 % HPGe detector.
Synthesis of HO-BD[BH3/H2O2/NaOH]: Graphite oxide (500 mg) was dispersed in 500 mL of anhydrous THF by ultrasonication (400 W, 60 min). 1 m BH3 solution (60 mL) in THF was added. The reaction mixture was heated at reflux under an argon atmosphere for 24 h. A mixture of 3 m NaOH/30 % H2O2 (1:1 by volume) was slowly added to the reaction mixture. It was subsequently heated at reflux for 3 h and the formed hydroxylated graphene was separated by a suction filtration and repeatedly washed with THF and water. The product was than dried in a vacuum oven (50 8C, 48 h).
Electrochemical voltammetric measurements were performed by using a microAutolab Type III electrochemical analyzer (Eco Chemie, The Netherlands). A NOVA 1.7 software (Eco Chemie) was used to control the electrochemical analyzer. The Nova 1.7 software was published by Metrohm Autolab B. V. to support of Metrohm Autolab’s potentiostat/galvanostat equipment. For the measurement of cyclic voltammetry the glassy carbon electrodes were renewed by polishing with alumina suspension to clean the electrode surface and then washed and wiped dry before each use. The materials were dispersed in DMF to obtain a 1.0 mg mL1 suspension. The suspension was then sonicated for 5 min at room temperatures prior to each use. A cleaned GC electrode was then modified by drip coating with a 1 mL aliquot of the suspension and left to dry under a lamp to obtain a layer of randomly dispersed material. The modified GC electrodes, Pt counter electrode, and Ag/AgCl reference electrode were then placed into an electrochemical cell that contained the electrolyte. The CV scans were then executed. The electrolytes used were 50 mm
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Synthesis of HO-TRG: Thermally reduced graphene was obtained by thermal exfoliation/reduction of graphite oxide prepared by the Hofmann method (HO-GO) at high temperature in a nitrogen atmosphere. Thermal exfoliation/reduction of graphite oxide was performed at 1000 8C and under pressure of 100 kPa. Graphite oxide (0.1 g) was placed in a porous quartz glass capsule connected to magnetic manipulator inside a vacuum tight tube furnace with a controlled atmosphere. The application of magnetic manipulator allowed us to create temperature gradient over 1000 8C min1. The tube was flushed with nitrogen and repeatedly
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Full Paper evacuated to remove any traces of oxygen. Then the reactor was repeatedly flushed with hydrogen and evacuated and the sample was then quickly inserted by a magnetic manipulator to a preheated segment of the furnace and held in there for 12 min. The flow of hydrogen during the exfoliation procedure was 1000 sccm to remove any evolved byproducts.
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Acknowledgements Z.S. and P.S. were supported by Specific University Research (MSMT No 20/2015) and the Czech Science Foundation (GACR No. 15-09001S). M.P. and H.L.P. acknowledge a Tier 2 grant from the Ministry of Education, Singapore (MOE2013-T2-1-056; ARC 35/13). Measurements of PGAA were carried out at the CANAM infrastructure of the NPI ASCR Rˇezˇ supported by the Ministry of Education, Youth and Sports (grant project No. LM2011019). Keywords: boranes · electrochemistry hydroboration · synthetic methods
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Chem. Eur. J. 2015, 21, 1 – 8
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These are not the final page numbers! ÞÞ
Received: November 21, 2014 Revised: March 10, 2015 Published online on && &&, 2015
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2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Full Paper
FULL PAPER & Graphene
Stand out from the crowd: By using a well-established hydroboration reaction chain it is possible to obtain highly hydroxylated compounds of precisely defined composition with a general formula (C1O0.78H0.75)n, named graphol and highly hydroxylated graphane (C1(OH)0.51H0.14)n (see figure). Compared with graphene, the compounds demonstrate enhanced electrochemical performance.
H. L. Poh, Z. Sofer, P. Sˇimek, I. Tomandl, M. Pumera* && – && Hydroboration of Graphene Oxide: Towards Stoichiometric Graphol and Hydoroxygraphane
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Chem. Eur. J. 2015, 21, 1 – 8
www.chemeurj.org
8
2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!
