Engineering a Water-Dispersible, Conducting, Photoreduced

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Engineering a Water Dispersible, Conducting, Photoreduced Graphene Oxide Bharathi Konkena, and Sukumaran Vasudevan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512759w • Publication Date (Web): 26 Feb 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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

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The Journal of Physical Chemistry

Engineering a Water Dispersible, Conducting, Photoreduced Graphene Oxide

Bharathi Konkena and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, INDIA

*Author to whom correspondence may be addressed. E-mail: [email protected]. Tel: +91-80-2293-2661. Fax: +91-80-2360-1552/0683;

ACS Paragon Plus Environment

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Abstract A critical limitation that has hampered widespread application of the electrically conducting reduced graphene oxide (r-GO) is its poor aqueous dispersibility. Here we outline a strategy to obtain water dispersible conducting r-GO sheets, free of any stabilizing agents, by exploiting the fact that the kinetics of the photoreduction of the insulating GO is heterogeneous. We show that by controlling UV exposure times and pH, we can obtain r-GO sheets with the conducting sp2-graphitic domains restored but with the more acidic carboxylic groups, responsible for aqueous dispersibility, intact. The resultant photoreduced r-GO sheets are both conducting and water dispersible.

Key Words: Graphene oxide, reduced graphene oxide, conductivity, zeta potential, pK, IR spectroscopy.


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INTRODUCTION Graphene sheets, one-atom-thick, two-dimensional layers of carbon atoms, have gained enormous importance over the past few years due to their unique attributes - high electronic and thermal conductivities and exceptional mechanical strength.1,2 These properties have led to the development of graphene based field-effect transistors,3 ultrasensitive sensors,4 and electromechanical resonators.5 Current procedures, such as mechanical exfoliation6 or chemical vapor deposition,7 are not ideal for the large-scale manufacture of processable graphene sheets and are unlikely to meet current requirements.8 The chemical reduction of suspensions of graphene oxide (GO) has emerged as a viable route for large scale production of graphene sheets.8,9 Over the years various procedures have been developed for the reduction of GO that include the widely used chemical reduction reaction using either hydrazine or sodium borohydride, plasma or thermally induced reduction and photochemical methods.8,10-12 Irrespective of the method of reduction the resultant reduced GO (r-GO) contains residual oxygen functionalities, holes and defects and consequently conductivities are considerably lower than that of graphene obtained by mechanical exfoliation.12 The conductivity, unlike in graphene where electrons and holes undergo ballistic transport, is by an activated mechanism.13 Nevertheless, r-GO is a versatile material with conductivities appreciably higher than that of GO and which can tailored over several orders of magnitude by controlling the degree of oxidation. Developing effective techniques to reduce graphene oxide as well as deciphering the underlying reduction mechanism are important both from a fundamental and applied perspective considering the number of potential applications. The light-induced reduction of GO, by exposure to UV radiation, is especially attractive, for apart from being rapid


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and facile the avoidance of hazardous chemicals makes it a ‘green’ procedure.14- 20 Recent studies have shed light on the mechanism of the photochemical transformation of GO to r-GO. In GO the sp2-bonded carbon network of graphite is strongly disrupted and a significant fraction of this carbon network is bonded to hydroxyl groups (C-OH) or participates in epoxide (C-O-C) groups with minor components such as carboxylic or carbonyl groups populating the edges of the GO sheets.21,22 In aqueous dispersions the photo reduction of GO on exposure to UV radiation has been shown, by pump-probe femtosecond spectroscopy, to be an indirect process, wherein the transformation to r-GO is initiated after the capture of solvated electrons, produced by the UV photoionization of water.23,24 It is the chemical potential of the photo-generated solvated electrons that drives the reduction of GO, and not simple heating effects. Earlier studies, too, had indicated that the UV light induced transformation of GO to r-GO in aqueous media is not a thermal event, but the mechanism suggested involved band-gap excitation and subsequent photo-catalytic reduction; the semiconductor domains of GO catalyzing its own photo reduction. The photoreduction of GO using a semiconductor photocatalyst such as TiO2 is well documented.25 It has also been established from laser induced photolysis of single GO sheets, that the photoreduction is both spatially and temporally heterogeneous.26 Reduction arises from the photoinduced migration and subsequent dissociation of hydroxyl groups located on the basal plane. The last step may also be accompanied by dissociation of carbonyl and carboxylic groups located at the edges of the GO sheets. These observations are in agreement with those of earlier studies that had found that CO2 evolution was much higher than that of H2 in the initial stage of the photoreduction but while the rate of CO2 evolution leveled off, the H2 production rate increased with increasing irradiation time.24 It is this heterogeneity that holds the key to


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developing new strategies to spatially control photoreduction and thus engineer the properties of the reduced GO. A major limitation that has hampered applications of r-GO is that, unlike GO, the sheets are hydrophobic, with limited aqueous dispersibility.27 This is true irrespective off the method of reduction. For example, during the photoreduction of aqueous dispersions of GO the photoreduced GO was found to agglomerate after 48 hrs of exposure due to increased hydrophobicity.24 Earlier studies using

zeta–potential measurements had

shown that the colloidal dispersions of both chemically reduced r-GO and GO are electrostatically stabilized, but whereas GO sheets are dispersible at pH values as low as 4.0 the chemically reduced r-GO sheets are dispersible only above pH 8.27,28 We had shown earlier from pK measurements that the superior dispersibility of GO as compared to the chemically reduced r-GO is because of the presence of ionizable groups on GO that are acidic (pK 4.3) in addition to groups with pK values of 6.6 and 9.8.28 On chemically reduced r-GO, on the other hand only carboxylic groups with pK 7.9 are present. In both GO and the chemically reduced r-GO sheets it is ionization of the carboxylic groups that is primarily responsible for the build up charge but on GO sheets the presence phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pK value by stabilizing the carboxylate anion through intramolecular hydrogen bonding, resulting in superior water dispersibility . The usual strategy to obtain water dispersible r-GO is by functionalization either by non-covalent or by covalent attachment of polymers, surfactants or cyclodextrin cavities to the sheets.29,30

Here we outline an alternate strategy to obtain water dispersible

conducting r-GO sheets, free of any stabilizing agent, by exploiting the fact that the kinetics of the photoreduction of GO is heterogeneous. We do so by controlling the


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photoreduction process so as to obtain a material wherein the sp2 network is partially restored, but with part of the carboxylic groups intact so as to obtain a material that is both conducting and water dispersible. Additionally, we have followed the progress of the elimination of the ionizable groups during photoreduction and identified the groups responsible for the water dispersibility of the conducting r-GO.

EXPERIMENTAL SECTION Preparation of Graphene Oxide (GO) and Reduced-GO. Graphene oxide (GO) was prepared from graphite by a modified Hummers method.31 In a typical reaction, 250 mg of graphite (Alfa-Aesar) and 46 mL of H2SO4 were stirred in an ice bath followed by slow addition of 6g of KMnO4. The solution was transferred to a water bath at 37oC, diluted, and 30% H2O2 added with stirring at which time the solution turns from dark brown to yellow. The mixture was allowed to settle and the precipitate washed till free from sulfate, re-dispersed and then centrifuged. The process was repeated till the dispersion was neutral. The chemically reduced r-GO which was used to compare the results of the photoreduced r-GO was prepared by dispersing 1.25 mg/mL of GO in 100 mL of water by sonicating followed by addition of 1mL of Hydrazine hydrate (32.1 mmol), and refluxing in an oil bath at 95oC for 24 hours at which time the solution turns black, and the reduced GO gradually precipitates as a black solid. For the photoreduction of GO aqueous dispersions were prepared by dispersing GO (1.2 mg/ml) in milliQ water followed by sonication for 2 hours. The photoreduction was carried out by illuminating the dispersions with a mercury vapor lamp (400 W; 17 mW/cm2) with ice-water circulated through a jacket condenser to prevent heating. The pH of the dispersion was


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maintained at a value of 4.0 and its temperature between 40˚C and 50˚C during photoreduction.

Physical Characterization. GO sheets and the 12-hour photoreduced GO were characterized by X-Ray Photoelectron and 13C CP-MAS NMR spectroscopy. XPS spectra were recorded on a Thermo Fisher Scientific Multilab 2000 Spectrometer using an Mg Kα source. The 13C CP-MAS NMR spectra of GO was recorded on a Bruker AV 500S – 500 MHz High resolution Multinuclear FT-NMR spectrometer at a spinning speed of 9.4 KHz. X-ray diffraction patterns were recorded using a Bruker-D8 Advance X-ray diffractometer (40 kV, 50 mA, sealed Cu X-ray tube) equipped with graphite monochromator. Atomic force microscope (Veeco MMAFMLN-AM-2113) images were recorded in the tapping mode. Raman spectra were recorded on a Horiba Jobin-Yvon Raman Microscope using 514 nm excitation. UV-Vis spectra of aqueous dispersions were recorded on a Perkin-Elmer Lambda35 spectrometer. FTIR spectra were recorded on a Bruker’s ALPHA-P FTIR spectrometer operating at 4 cm-1 resolution. Steady state fluorescence spectra were recorded on a Horiba Jobin Yvon Fluoromax-400 spectrophotometer. Excitation and emission slit widths were set to 7 nm, with a typical scan speed of 120 nm/min. For recording spectra at different values of pH the photo reduced samples were collected from the titration experiment at select values of pH. Zeta potentials of aqueous dispersions, at different values of pH, were determined using a Zetasizer Nanoseries-ZEN 3690 (Malvern) instruments. The I-V measurements were performed in scanning tunneling microscope (STM) mode using a freshly prepared tungsten metal tip. Samples were prepared by spin coating the aqueous dispersions on a HOPG substrate. The sheet dimensions were obtained from Atomic Force Microscope


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images recorded in the tapping mode (see Supporting Information Figure S3). GO sheets have typically thickness of 1.1 nm while the 12 hour photoreduced GO has thickness of 0.48 nm. The thickness is as expected for single sheets of GO and reduced GO respectively. Details of the characterization of GO and photoreduced r-GO are provided as part of the Supporting Information.

RESULTS AND DISCUSSIONS The photoreduction of GO dispersions was carried out by exposing aqueous dispersions of GO to UV, while maintaining a constant pH of 4.0. On UV exposure the color of the dispersion changed from brown to black and after 48 hours the r-GO sheets agglomerated and were deposited at the bottom of the container. The conductivity of the GO sheets at different stages of photoreduction was monitored by drawing out samples at different exposure times and depositing, by spin-coating, on a HOPG substrate. The current (I) - voltage (V) curves of GO nanosheets at different irradiation times measured using an STM with freshly prepared tungsten tips, are shown in Figure 1a. For comparison the I-V curve of the chemically reduced r-GO is also shown. It may be seen that the conductivity of the GO sheets increases with irradiation time, reaching a value of 2.2 × 103 S m-1 on irradiation for 2 hours. The conductivities at this stage of photo exposure are comparable to that of the chemically reduced r-GO (2.1 × 103 S m-1) and is 5-orders of magnitude greater than that of the starting GO. Above 2 hours of exposure the change in conductivity is, at best, incremental and it may be seen that the conductivities after 2 hours of exposure and the agglomerated r-GO obtained after 48 hours exposure are almost identical. The conductivity data suggests that most of the sp2 domains, that are


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(a)

(b)

(c)

Figure 1. (a) I-V curves of GO sheets recorded after different UV- irradiation periods. For comparison the I-V curves of graphite (HOPG) and chemically reduced r-GO are also shown. (b) Photograph sequences of aqueous dispersions of photo-reduced r-GO obtained after an irradiation period of 12 hours and chemically reduced r-GO at different values of pH. (c) UV-visible spectra of GO aqueous dispersions recorded at different irradiation intervals.

in fact responsible for the conductivity, are restored within 2 hours of UVphotoreduction. The flattened portion of the I-V curve at low bias in Figure 1a is because of defects in the sheet at the transition contact point that function as a Schottky junction.32 What is remarkable about the data of Figure1a is that at exposure times for which the conductivities are comparable to that of the chemically reduced r-GO the photoreduced r-GO is water dispersible, whereas the chemically reduced material is not. This contrasting behavior is highlighted in Figure 1b which shows that the photoreduced GO is


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dispersible for all pH values above 2, whereas the chemically reduced GO is dispersible only above a pH value of 8. The aqueous dispersibility of the 12 hour photoreduced GO is, in fact comparable with that of the starting GO. Corroborative evidence that the sp2 network is restored at short exposure times is provided by the optical absorption spectra of the dispersions recorded after different exposure times (Figure 1c). The UV spectrum of the starting GO solution shows two bands; a maximum centered at 225 nm that can be assigned to the π-π* transition of aromatic C=C bonds, and a shoulder at 300 nm corresponding to the n-π* transition of the C=O bonds.27 On exposure to UV, photoreduction causes a partial restoration of C=C bonds, resulting in a shift in the absorption maximum from 225 nm to 250 nm. It may be seen that for exposure times longer than 2 hours there is no further shift or change in the absorption spectra. These results, like the conductivity data, indicate that the formation of the sp2 domains, responsible for the conductivity, is complete within the initial phase of the photoreduction. In order to understand the origin of the aqueous dispersibilty of the 12 hour photoreduced conducting r-GO we have carried out zeta potential (δ) measurements at different values of pH and compared the results with that of the chemically reduced r-GO (Figure 2a). The conductivities of the two samples are similar. The zeta potential is an important factor for characterizing the stability of colloidal dispersions and provides a measure of the magnitude and sign of the effective surface charge associated with the double layer around the colloid particle. Generally, particles with zeta potentials more positive than +30 mV or more negative than −30 mV are considered to form stable dispersions due to interparticle electrostatic repulsion.33 For the 12-hour photoreduced rGO sheets, the δ potential drops below -30 mV above a pH value of 3, developing


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(b)

(d)

(a)

(c)

Figure 2. (a) Zeta potentials of photo-reduced and chemically reduced r-GO as a function of pH. (b) The concentration of ionizable groups as a function of pH for GO dispersions photo-reduced for different UV irradiation periods. (c) the corresponding pK distribution of the acid groups. (d) Variation in the concentrations the acid groups with different pK as a function of the irradiation time.

sufficient negative charge to form stable dispersions. The variation of the δ potential with pH for the chemically reduced r-GO sheets is quite different, dropping below -30 mV only above a pH value of 8 and it is only above this value that stable dispersions are obtained. The results for the chemically reduced r-GO is similar to that reported earlier.27,28 The results of Figure 2a suggest that stability of the aqueous dispersions of the photoreduced r-GO is a consequence of the negative charge on the sheets. This charge develop because of the ionization of the functionalities present and suggest that the nature of the ionizable groups present on the photoreduced and chemically reduced r-GO sheets are quite different.


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The concentration of acid groups on the photoreduced r-GO and their pK distribution at different exposure times was determined by acid-base titrations (see Supplementary Information).28 The concentration of ionizable groups as a function of pH, per gram of the photoreduced r-GO, is shown in Figure 2b. The concentration of ionized species decreases with irradiation time clearly indicating significant removal of oxygen functionalities on photo-exposure. The concentration profiles show multiple inflections that indicate the presence of acid groups with different dissociation constants. To identify the pK values of the different functionalities and how their relative concentrations vary with exposure times we differentiate the concentration profiles with respect to pH (Figure 2c). The acidity distribution of GO shows three broad peaks indicating the presence of three different acid groups with pK values of 4.3, 6.6 and 9.8 with relative contributions in the ratio 3.0: 7.4:6.5. These features had been assigned previously; groups with pK 4.3 are assigned to carboxylic groups that are in close proximity (one-bond removed) to a hydroxyl group while the pK values 6.6 and 9.8 are assigned to isolated carboxylic and hydroxyl groups (see Scheme 1).28 It may be seen that on photo-exposure the contribution of groups with a pK value of 6.6 decreases rapidly and so also groups with pK 9.8. The groups that survive even after long illumination times are the groups with pK value 4.3. This trend is more clearly seen in Figure 2d where the progress of the photoreduction of the ionizable groups on GO has been monitored by plotting the variation of concentration of different acid groups present on GO as a function of illumination times. The concentrations at the start represent the relative concentrations of the three acid groups present on the GO sheets, prior to photo-exposure. It may be seen from Figure 2d that the concentration of acid groups with pK 6.6 and 9.8, drop rapidly within a short interval of irradiation. Groups with pK 9.8 (the isolated hydroxyls) are completely absent after


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Scheme 1 exposures for 1 hour while isolated carboxylic (pK 6.6) are absent after 2 hours. The acid groups with pK 4.3, however, survive for much longer exposure times. Even after a 12 hour exposure there is a significant concentration of these groups and it requires at least 48 hour exposure before they are completely removed. Since these are the only acid groups present on GO for exposure times greater than 2 hours they must be the groups responsible for the aqueous dispersibility of the conducting photoreduced r-GO. In contrast the only ionizable groups present on the chemically reduced r-GO are those with a pK value of 7.9. The fluorescence spectra of GO dispersions at different exposure times, on excitation at 320 nm, are shown in Figure 3. With increasing exposure time the fluorescence intensity decays, rapidly for exposure times less than 60 minutes and then remains roughly constant till ~ 48 hours when flocculation occurs (inset of Figure 3). The changes in the fluorescence spectra of GO on photoreduction are better understood from the normalized spectra. Prior to photoreduction GO dispersions show two bands when excited at 320 nm; a blue emission band at 425 nm and a emission band in the green at 540 nm. The spectra are similar to that reported earlier.34 The green emission at 540 nm


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Figure 3. Normalized fluorescence spectra of GO aqueous dispersions at different irradiation times at an excitation wavelength of 320 nm. The inset shows that as-recorded spectra.

is characteristic of carboxylic groups in their protonated state while the blue emission at 425 is due to dissociated carboxylic groups. It is well known that dissociation of the carboxylic group in aromatic carboxylic acids raises the energy of the first singlet state causing a blue shift.35 In the present experiments the pH of the dispersions was maintained at 4.0. The green emission band at 540 nm may therefore be identified with acid groups with pK 6.6 that remain protonated at this pH, while the band at 425 nm may be assigned to the dissociated acid groups with pK 4.3. The latter are the carboxylic groups that have a hydroxyl group at an adjacent ortho position. On UV-exposure there is a sharp decrease in the intensity of the 540 nm band and after 60 minutes of exposure is almost absent. The observation is in agreement with the


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pH titration data that had indicated that the concentration of groups with pK 6.6 is negligible after 60 minutes of UV exposure. The blue emission shows an apparent red shift from 425 nm to 450 nm after an exposure time of 60 minutes. It may, however, be seen that the band at 450 nm persists with no change in emission maxima even for long exposure times. It is also seen in the as recorded spectra shown in the inset of Figure 3. This band had been observed and reported in earlier studies of the photoreduction of GO.36 The origin of this band had been ascribed to disorder induced defect states within the π-π∗ gap of the newly formed sp2 graphitic domains that are restored by the removal of oxygen-containing functional groups. The electron-hole recombination among these sp2 states exhibits blue fluorescence with a narrower distribution.36,37 The blue emission observed in the present study is likely to have a similar origin. In the earlier reports the changes in the fluorescence spectra on photoreduction had been characterized as a blue shift due to an increase in the size of the graphitic domains.36 Our explanation differs. In the light of the changes observed in the pH titrations on UV-exposure of GO dispersions, described in the earlier section, we interpret the changes in the fluorescence spectra as due to the elimination of the green emitting carboxylic fluorophores during the initial stages of photoreduction and the subsequent blue emission from defect states within the π-π∗ gap of the newly formed sp2 graphitic domains. It is however difficult to monitor the fate of dissociated carboxylic groups that emit at 425 nm during photoreduction because of overlap with 450 nm emission band originating from the restored graphitic domains. The infrared spectra, however, allows for these two events to be monitored with greater clarity. The progress of the photoreduction of the ionizable oxygen functionalities on the GO sheets was followed by recording the infrared spectra after different photo-exposure times


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(a)

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(b)

Time (min)

Figure 4. (a) FT-IR spectra of GO recorded after different UV- irradiation intervals. (b) Variation of the normalized intensity of the 1730 cm-1 and 1430 cm-1 bands as a function of the irradiation time.

(Figure 4a). It may be seen that there are significant changes with exposure time; these regions have been highlighted in Figure 4a. For GO sheets, prior to UV illumination, the assignment had been reported earlier.28,38 The intense band at 1730 cm-1 (designated as the α region in Figure 4) is assigned to the carbonyl (C=O) stretching mode of the undissociated carboxylic (-COOH) group, the band at 1617 cm−1 (designated as the β region) is due to the bending mode of H2O molecules that hydrate the more acidic carboxylic groups.28,39 The band at 1251cm−1 is due to the phenol/hydroxyl -C-OH stretch and the band at 1060 cm−1 is due to the -C-O-C stretch of the epoxide groups. The band at 1730 cm−1, has contributions from carboxylic groups with pK 6.6 and 4.3. It is, however, only the latter that is hydrated; the band at 1617 cm−1 is due to water associated with the more acidic carboxylic group.39 On photo-exposure the intensity of the 1730 cm−1 band


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initially drops rapidly but levels off after about an hour and it is only on long exposure that it disappears completely. It may however be noted for short exposure times the intensity of the 1617 cm−1 water bending mode shows little change. At longer exposure times it is difficult to monitor the intensity of the 1617 cm−1 band because of the appearance the bands at 1430 cm-1 and 1598 cm-1 (designated as the γ region) that may be assigned to the

C=C stretching modes of the aromatic segments of the sp2

network.28,40 The bands at 881 cm-1 and 760 cm-1 (designated as the ε region), that appear after about 15 minutes of exposure may be assigned to vinylidene C-H out-of-plane bending modes.40 The normalized intensities of the 1730 cm−1 carboxylic stretching band and the 1598 cm-1 C=C stretching modes of the aromatic segments have been plotted as a function of photo-exposure times in Figure 4b. It may be seen that after 2 hours of UV illumination the intensity of the 1598 cm-1 shows no further increase indicating that the maximum possible restoration of the sp2 network has already occurred. At these exposure times, however, carboxylic groups are still present on the sheets. Although the intensity of the 1730 cm−1 carboxylic stretching band drops rapidly during the initial exposure times it levels off after 2 hours and drops to zero only after 48 hours of UV exposure. The intensity profiles in Figure 4b clearly indicate that the kinetics associated with the restoration of the conducting sp2 domains and the elimination of the carboxylic groups during photoreduction are very different. For the different carboxylic groups, too, the reduction kinetics is different. The fact that the water band persists indicates that the more acidic carboxylic groups, which are hydrated, are more difficult to reduce.41 These results are in agreement with those obtained from pH titrations that had indicated that it is the


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more acidic groups with pK 4.3 that are difficult to reduce as compared to acid groups with pK 6.6. The above results highlight the role of acid groups with pK 4.3 in ensuring the aqueous dispersibility of the conducting photoreduced r-GO. These groups are carboxylic groups that have a hydroxyl (-OH) in close proximity. The dissociated carboxylate anion is stabilized by intramolecular hydrogen bonding and consequently these groups are more acidic with lower pK values as compared to isolated carboxylic groups. There are multiple evidences for the presence of carboxylic groups with hydroxyl groups in the ortho position on the GO sheets. For example, the observation of excited state intramolecular proton transfer - the hydroxyl proton to the carboxylate anion – in the time resolved fluorescence spectra.34 Additional evidence for carboxylic and hydroxyl groups in close proximity on the periphery of the sheets comes from DFT calculations that showed that H-bonding acts to keep the two groups adjacent to one another, with a calculated interaction energy between the two groups of about 7.0 kcal/mol.42 Since photo-reduction is initiated by photogenerated hydrated electrons these acid groups that remain in the dissociated anionic state, under the present experimental conditions, are unreactive and difficult to reduce.

CONCLUSIONS In conclusion we have shown how the properties of GO can be engineered to obtain a material that is both conducting and dispersible in aqueous media. We do so by the UVlight induced reduction of GO, which unlike the chemical reduction process, offers a greater degree of control. Here we exploit the fact that the photoreduction of GO is both spatially and temporally heterogeneous and show that kinetics associated with the


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restoration of the sp2 domains, responsible for the conductivity, and the elimination of the carboxylic groups, responsible for aqueous dispersibility, are very different. In particular the acidic carboxylic groups (pK ~ 4.3) located on the perimeter of the GO sheets and have a hydroxyl at an adjacent position, which stabilizes the dissociated carboxylate anion by intramolecular hydrogen bonding, are resistant to elimination by photoreduction. It is, therefore, possible by controlling the UV illumination times to obtain reduced GO sheets wherein the maximum possible restoration of the sp2 domains has been achieved but with the carboxylic groups with pK 4.3 intact. We show here by limiting the UV illumination of aqueous dispersions of GO to short exposure times we can obtain a material with conductivities comparable with that of the chemically reduced hydrophobic r-GO but with aqueous dispersibilities similar to that of GO. An immediate application would, of course, be water-based conducting paints and inks but we envisage a host of new applications for the water dispersible, conducting, reduced graphene oxide sheets.

ASSOCIATED CONTANT Supporting Information: (S1) XPS and 13C CP-MAS NMR spectra of Graphene Oxide and photo-reduced r-GO. (S2) XRD and Raman Spectra of GO and Photoreduced r-GO. (S3) AFM images of GO and the 12 hour photoreduced r-GO. (S4) pH Titration method. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest


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Engineering a Water-Dispersible, Conducting, Photoreduced

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