Reaction Kinetics of Reducing Graphene Oxide at Individual Sheet

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C: Physical Processes in Nanomaterials and Nanostructures

Reaction Kinetics of Reducing Graphene Oxide at Individual Sheet Level Studied by Twilight Fluorescence Microscopy Katsuki Kanazawa, Hikaru Sato, and Masahito Sano J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12235 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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The Journal of Physical Chemistry

Reaction Kinetics of Reducing Graphene Oxide at Individual Sheet Level Studied by Twilight Fluorescence Microscopy

Katsuki Kanazawa, Hikaru Sato, and Masahito Sano*

Department of Organic Materials Science, Yamagata University Yonezawa, Yamagata 992-8510, Japan

ACS Paragon Plus Environment

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ABSTRACT

Kinetics of the reduction reaction of graphene oxide (GO) by L-ascorbic acid in solution was examined by direct observation of an individual GO sheet with an area over 100 m2 using Twilight fluorescence microscopy. Temporal changes of either increase in absorbance or decrease in autofluorescence from an individual GO sheet of a known layer number and an area were analyzed by the pseudo-first order model, the pseudo-second order model, and the Elovich model. An increase in the absorbance around 600 nm probes an extension of the -electron conjugation length over multiple hexagonal units. All data at various conditions are best fitted with the Elovich model, implying that the activation energy increases as the reaction progresses. A decrease in the total autofluorescence intensity follows local deoxygenation of oxides. In this case, the pseudosecond order model, which is based on a homogeneous surface with a constant activation energy, shows the best fit to all data. These results demonstrate that, for GO sheets containing many kinds of oxides at various binding sites, different kinetics appear depending on which elementary reactions we follow. For both cases, the rate constant was independent of a number of layers but became larger for smaller area sheets. The area dependence is explained by the oxidation characteristics of the Hummers’ method and the sonication process to exfoliate the oxidized graphite.


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Introduction Ideal graphene consists of a single-atomic layer of the carbon hexagonal sheet that affords highly extended -electron conjugation. Although graphene offers excellent electrical, thermal, and mechanical properties, its poor dispersibility and reactivity pose significant problems in chemical applications. Additionally, while various techniques to synthesize a large-area singlelayer graphene sheet on a solid surface have been developed for applications in dry processes, its total mass is too small to be useful in wet processes. One of the methods to overcome these problems is to oxidize graphite flakes to yield graphene oxide (GO).1,2 For instance, the Hummers’ reaction can produce oxidized graphite flakes in mass quantities.3 The resulting oxidized graphite flakes are exfoliated easily by sonication to yield GO sheets, which are readily dispersible in various polar solvents and can be chemically modified as an organic substance. The Hummers’ reaction, however, involves violent oxidation processes that are difficult to control. It not only produces oxide groups with various chemical structures and concentrations at different binding sites on the hexagonal sheet but also cleaves carbon-carbon bonds in unpredictable ways. The original -electron conjugation is severely damaged and, as a result, the attractive physical properties of graphene are mostly lost. In an attempt to restore conjugation, GO is chemically “reduced” in a sense that the oxygen content of GO is decreased.4 It should be noted that the actual reduction here is not a simple organic reaction against well-defined functional groups. Considering chemical and structural complexities of GO produced by the Hummers’ method, present understanding of the reduction reaction remains as poor as that of the Hummers’ oxidation.


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Reaction kinetics is one of the fundamental studies to characterize chemical reactions. In ordinary studies, concentrations of either well-defined reactants or products are followed in time and compared with various theoretical models. In the case of GO, in addition to chemical heterogeneity discussed above, a presently available sample is a mixture of GO sheets with various areas, layers, and different degrees of oxidations, which may influence the reduction kinetics. It is necessary to follow the reaction of a particular GO sheet in real time. Recently, we developed Twilight fluorescence (TwiF) microscopy which is capable of directly imaging a single layer graphene compound floating in liquid.5 In a standard procedure, a fluorescent dye is added to a GO solution. TwiF microscopy utilizes a unique optical configuration so that a small modification of the dye fluorescence by GO produces large enough contrast to visualize a singlelayer GO sheet floating in the solution. If a reaction induces a change in any fluorescence properties associated with GO, kinetics can be followed at the individual sheet level. Because the reduction reaction involves numerous kinds of oxide species located at different binding sites as well as various modifications of the original -electron conjugation, it can be considered as a simultaneous occurrence of many elementary reactions in close proximity. Then, we expect to see different kinetics depending on which elementary reactions we follow. In this study, we follow two different kinds of fluorescence modification. The location of observed GO sheets in solution is chosen so that TwiF contrast is mostly due to light absorption by GO. Reduction of GO increases its absorbance and GO appears darker as the reaction proceeds. The employed wavelength is around 600 nm, implying that kinetics here is related to reactions involving an extension of -electron conjugation. In the second experiment, we follow autofluorescence of GO. GO itself in suitable solvents fluoresces under UV excitation. The same TwiF microscope is used without the addition of dye. Although the origin of autofluorescence is


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still under investigation, oxides and local perturbations to -electron conjugation are known to be responsible.6 Kinetics in this case is related to local deoxygenation. In each case, we examine dependences on the area and the number of GO layers in a sheet. The reduction reaction of GO was first examined as a macroscopic mixture containing various kinds of GO sheets. Traditional spectroscopic techniques, such as UV-Vis absorption spectroscopy, Raman spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), were applied to obtain ensemble-averaged properties of the reacted GO. Then, two different experiments using TwiF microscopy were performed to follow the reduction kinetics of an individual sheet.

Experimental Methods Reduction of GO. GO was synthesized by the original Hummers’ method3 using graphite flakes (Ito Kokuen Kogyo, Z-100). It was dispersed in N-methyl-2-pyrrolidone (NMP) and exfoliated by sonication for approximately 5 s in a bath sonicator (Bransonic, 2510J-DTH). The temperature of all solutions and the microscope cell was maintained at 50 °C throughout the reaction. A very small amount of GO dispersed in an NMP solution of rhodamine-6G (R6G, concentrations = 170, 330 M) was mixed with an NMP solution of L-ascorbic acid (L-AA, 10, 20, 30 mg/mL), and the mixture was set immediately in the microscope cell for observation. Since we needed to follow an individual sheet, the concentration of GO was unmeasurably small to ensure that each sheet was clearly separated. The same procedure was taken without R6G for autofluorescence (L-AA = 50 mg/mL).


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Ensemble-Averaged Characterization. UV-Vis absorption spectroscopy (V-570 spectrophotometer, Jasco) was performed with a reaction mixture in a 1.0 cm square cuvette. For other spectroscopies, a small portion of the reacting aqueous solution without dye was extracted at a controlled time and GO sheets were washed on a filter paper immediately. The dried sheets were analyzed by Raman spectroscopy (Almega XR, Thermo Fisher Scientific) operating with a 532 nm laser and ATR-FTIR (Nicolet iS20, Thermo Fisher Scientific). TwiF Observation. TwiF microscopy was operated with the excitation wavelength at 510 < exc < 560 nm and the emission wavelength at 590 nm < em for R6G, 330 < exc < 380 nm and 420 nm < em for autofluorescence. The sheet area was directly measured from the image. Because GO sheets were partially transparent, the number of layers was directly countable from the overlapped section for the sheet thinner than 3 layers. For thicker layers, counting was no longer possible and the unknown layer number was indicated as “> 4”.

Results and Discussion Ensemble-Averaged Chemical Changes. As the reaction continued, a solution color changed from brownish to black. Reduction of GO by L-AA has been reported by a number of groups and its deoxygenation efficiency is comparable to that of hydrazine.7-9 It should be noted that our GO sheets have the area ranging from 100 m2 to 3000 m2, whereas others have employed submicron-sized sheets. The large area reduced light scattering in the UV-Vis region. TwiF microscopy also showed that the reduced GO stayed dispersed as individual sheets in NMP. Thus, darkening of the color indicates an increase of visible absorption by each sheet. The large area also implies that a contribution from the sheet edges can be neglected.


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(b)

(a)

3400

OH C=O

COC

1620

1230 1730

1380

1070

Peak Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

OH / C=O

1 COC / C=O

4000

3000 2000 -1 Wavenumber (cm )

1000 0 0

OH / COC 200 400 Reaction Time (sec)

600

Figure 1. (a) ATR-FTIR spectrum of GO and peak assignments. (b) Peak ratios of oxide groups at different reaction times.

UV-Vis spectroscopy, which should reveal the result of reduction around 260 nm, did not give useful information due to the strong absorption of NMP below 300 nm. The use of NMP was necessary to keep GO sheets dispersed individually after reduction. We also found that Raman spectroscopy was not useful to follow the reaction because the D band intensity did not change more than the experimental error. This can be understood by noting that the present reduction is essentially a deoxygenation reaction. Figure 1b plots a temporal change of the ATR-FTIR peak ratios by the reaction. The hydroxyl and epoxy groups decrease faster compared with the carbonyl group, which is consistent with other reports.7, 8 The decreasing rates are about the same for both groups. TwiF Microscopy with Dye. In TwiF microscopy,5 GO is dispersed in a highly concentrated dye solution and the mixture is placed on the glass surface. The excitation beam is incident on


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the glass/mixture interface through an objective lens located on the glass side with an angle that has a small width around total internal reflection. Both the evanescent field and the faint refracted beam enter the dye mixture and decay exponentially with the characteristic lengths on the order of 200 nm and 2 m, respectively. The fluorescence excited by both lights is collected by the same objective lens on the glass side. Contrast  is defined as (I0 – I)/I0, where I0 is the background intensity reaching the lens from a pure dye solution region and I is the light intensity passed through a graphene sheet. The intensity I can be decreased by GO through two mechanisms; absorption and Förster resonance energy transfer (FRET). Viewed from the objective lens, dye molecules that are located behind a GO sheet receive less excitation intensity due to GO’s absorption. The emitted light is also diminished when it passes through the GO sheet on the way back to the lens. To quench fluorescence by FRET, dye molecules need to be located within about 10 nm from the GO surface.10 It implies that FRET can contribute to the TwiF contrast when the dye is excited by the evanescent field having the short decay length. This situation occurs only when a GO sheet is located very close to the glass surface. In the present experiment, moving the focal plane perpendicular to the glass surface indicated that the measured GO sheets stayed still at a distance farther than a few m away from the glass surface. Therefore, FRET does not enter and the measured contrast is due to light absorption by GO. In the present case of concentrated R6G, the excitation and emission wavelengths are around 540 and 605 nm, respectively.5 Since GO has a relatively constant absorbance around this region, we approximate the absorbance of excitation and emission wavelengths of GO nearly equal. Then, for 𝑛-layer graphene with a transmittance 𝑇 , the contrast 𝜂 is given by


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Figure 2. TwiF images of GO sheets in the reacting solution with 167 M R6G and 20 mg/mL L-AA. The reaction of each piece indicated by a circle is analyzed separately. The right image is 3600 sec after the left image.

𝜂 = 𝜂0(1 ― 𝑇2𝑛)

(1)

The contrast increases (appears darker) as either the absorbance increases (the transmittance decreases) or the number of GO layer increases. Figure 2 shows a change of the TwiF image during the reaction. More sheets become visible at a later time due to increases in the TwiF contrast. Also, some sheets have drifted in or out during the reaction. We analyzed only the stationary sheets, which meant that the distance from the glass surface had remained constant. Figure 3a presents TwiF spectra from a single GO sheet floating in the reaction mixture at different reaction times. These spectra are identical to that of concentrated R6G in NMP solution. Only the total intensity is decreased without any change in the spectral shape, confirming that the reaction only affected the GO absorbance. A typical temporal change of contrast  on a single GO sheet is shown in Figure 3b. In the present experiment, the oxides existed only on the GO surface and the GO concentration was kept unmeasurably small to image individual sheets. Thus, L-AA was present in far excess and


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(a)

(b) 0.6

immediately after

Elovich 1st order

240 sec 1200 sec

Intensity (a.u.)

2nd order

1800 sec

600



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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650 700 Wavelength (nm)

750

0.4

0.2 0

500

1000 1500 Time (sec)

2000

Figure 3. (a) Temporal change of TwiF spectra from a single GO sheet during the reaction. (b) Temporal change of TwiF contrast on a single GO sheet in the reacting solution with 330 M R6G and 20 mg/mL L-AA. The least-square fit to the pseudo-first order (black, dotted curve), the pseudo-second order (blue, solid), and the Elovich (red, solid) are shown.

the data were analyzed by the following three kinetic models written in an integrated product form. The pseudo-first order model η = 𝜂0(1 ― 𝑒 ― 𝑘1𝑡)

(2)

The pseudo-second order model

η=

𝑘2𝑡𝜂02 1 + 𝑘2𝑡𝜂0

(3)


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The Elovich model η = 𝜂0 + 𝑘𝐸𝑙𝑜𝑔(𝑡)

(4)

where 𝑘1, 𝑘2, 𝑘𝐸 are the rate constant for each model. For all experiments in different conditions, the pseudo-first model does not fit any data at all, implying that the reaction is not against a single oxide species. The Elovich model fits all data well (the average residue of all data R = 0.954), whereas the pseudo-second order model fits only some data (R = 0.920). Due to macroscopic sizes of the GO sheets, the present reaction is closely related to adsorption. The Elovich model is often applied to adsorption of dye11 and metals12, 13 on heterogeneous surfaces. Also, it has been applied in adsorption of various substances on GO.14-17 The model is based on irreversible adsorption on a heterogeneous surface with a characteristic that the activation energy increases as the adsorbate concentration increases on the surface. This energy dependence makes the Elovich model fundamentally different from other models where the energy remains constant. In the present case of GO reduction, the increasing activation energy may be caused by the presence of various oxide groups with a wide spectrum of energies produced initially by the Hummers’ reaction. The activation energy differs not only by the kind of oxides but also on their binding sites on the graphene plane. Since the reaction starts from the oxides with low activation energies, ones with higher energies are left behind till later times. Also, it is conceivable that the reduction of an oxide stabilizes neighboring oxides. As the reaction progresses, a certain fraction of remaining oxides increases their activation energies. This scheme requires specific arrangements of chemical structures, causing the concentration of oxides satisfying this condition small. On the other hand, its rate constant can be very large since it only involves electron transfer through conjugated bonds. Thus, both schemes can play an


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important role in the kinetics. As a result, the Elovich model implies that the reduction reaction of GO will never be completed nor reach a maximum yield even if the reaction is carried out for a very long time. Furthermore, the absorbed wavelength is around 600 nm. A number of theoretical calculations on quantum wells indicate that it requires 2D carbon clusters with over 20 hexagonal units to produce the band gap of this energy.18, 19 For many small aromatic compounds with oxide side groups, this energy corresponds to a -electron conjugation length of over 3 benzene units. Thus, the quality of -electron conjugation to which an oxide binds becomes an important factor to increase the absorbance. Reduction of a single oxide group that attaches to relatively undamaged parts of the hexagonal sheet may be sufficient to increase local absorbance. But for heavily damaged parts, many oxide groups within neighboring multiple hexagonal units need to be reduced. The Elovich model indicates that the low activation energy oxides are gathered in close proximately. In other words, some areas consisting of many hexagonal units are reduced easily (a)

(b) 0.50

0.50

0.25

0.25

0.0 0

kE

kE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1000 2000 Area (m2)

3000

0.0

1

2 Layer

3

>4

Figure 4. Dependence of the Elovich rate constant on (a) area and (b) a number of layers. ▲ R6G = 167 mM/ L-AA = 20 mg/mL) , ● (330/10), ■ (330/20) ◆ (330/30).


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while other areas are not. This heterogeneous area characteristic has been seen in the photolysis of GO also.20 Dependences of the Elovich rate constant on the area and layer number are given in Figure 4. Different L-AA and R6G concentrations were examined. For each set of the concentrations denoted by the same color and symbol in the figure, multiple sheets from a single TwiF window were followed, meaning that these sheets had been subjected to the same reaction condition. Within each set, there is a tendency that smaller area sheets have a larger rate constant. The sheets belonging to the (330/10) set happen to have similar areas that this trend is hidden within the deviation. Furthermore, although different sets refer to independent reactions, all data together also show the same tendency. We shall discuss this point later. Contrary, there is no dependence on the layer number. Considering over 100 m2 sizes of our GO sheets, we think that each layer is adhered with the neighboring layers only at a few places and also has many holes. There are large enough spaces between layers that allow L-AA molecules to diffuse in freely. Thus, each layer can react with an independent rate. The data as a whole indicate that there is no systematic dependence on the L-AA and R6G concentrations. Small random differences between sets are probably due to different degrees of oxidation by the Hummer’s reaction.


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Figure 5. TwiF autofluorescence (monochromatic) image of GO in NMP.

TwiF Microscopy with Autofluorescence. GO itself is known to fluoresce under UV excitation.18-30 This autofluorescence allows imaging a single layer GO sheet by the same TwiF microscope without the addition of dye, as shown in Figure 5. In this case, GO is the only fluorescent species. The excitation beam incidents directly on the GO surface and the emitted light travels to the lens directly. Thus, the autofluorescence intensity from each GO sheet can be directly followed without referring to the solution part. Figure 6a exhibits a temporal change of the autofluorescence spectra from a single GO sheet during the reaction. It is characterized by a band around 670 nm with broad multiple peaks. In the literature, two kinds of spectra have been reported; one similar to ours with the band around 670 nm and another with an additional band around 470 nm. Some reports point out that two distinct bands are caused by different functional groups25-29 while others argue on electronic perturbations to -electron conjugation18,23,24 and formations of excimer30. Actually, we have observed a spectrum with two bands from a few GO sheets, but the majority of sheets exhibit a single band around 670 nm. In any event, as the reaction proceeds, the total intensity decreases


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(a)

(b)

reaction time (sec) ~0 300 600 900 1200

[AF] (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Elovich 1st order 2nd order

450

600 750 Wavelength (nm)

900

0

500

1000 1500 Time (sec)

2000

Figure 6. (a) TwiF autofluorescence spectra of a GO sheet in the reacting solution. (b) Temporal change of the total autofluorescence intensity from a single GO sheet in the reacting solution. The least-square fit to the pseudo-first order (black, dotted curve), the pseudo-second order (blue, solid), and the Elovich (red, solid) are shown.

without changing the spectral shape. The total autofluorescence intensity [AF] was analyzed by the kinetic models written in an integrated reactant form. The pseudo-first order model [𝐴𝐹] = [𝐴𝐹]0𝑒 ― 𝑘1𝑡

( + [𝐴𝐹]1)

(5)

The pseudo-second order model

[𝐴𝐹] =

[𝐴𝐹]0 1 + [𝐴𝐹]0𝑘2𝑡

(6)

The Elovich model


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[𝐴𝐹] = [𝐴𝐹]0 ― 𝑘𝐸𝑙𝑜𝑔(𝑡)

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(7)

It should be understood that the rate constants 𝑘1, 𝑘2, 𝑘𝐸 are numerically different from those of the TwiF experiment with R6G. Figure 6b presents a typical kinetic behavior. Compared with the TwiF experiment with R6G (Figure 3b), [AF] changes faster than . [AF] decreases rapidly for the first 1000 s, then tends to level off. However, it does not stay constant and keeps decreasing very slowly (about 5% in 1000 s). A mathematically optimized exponential function to this kind of a curve deviates most at the region of a large gradient due to the inclusion of the slowly decreasing part. To avoid this technical problem in the pseudo-first order fitting, a constant term was added (the parenthesis term in equation (5)). Both the 3-parameter pseudo-first order model and the pseudo-second order model fit well except the longtime regime. The Elovich model does not fit at all. Because the pseudo-second order model nearly reproduces the whole reaction time without adding a further parameter, we conclude that the kinetic is best described by the pseudo-second order model. As discussed above, the autofluorescence of GO does not originate from a simple process and likely involves many mechanisms. Nevertheless, all mechanisms are based on oxides and their local electronic perturbations to -electron conjugation. Some oxides and local perturbations are removed by the reaction, but the extent of removal should depend on the kind of oxides and electronic structures of binding carbons with a wide spectrum of the activation energy. On the other hand, the pseudo-second order model is based on a homogeneous surface with constant activation energy. One of the explanations to this apparent discrepancy is the high sensitivity of the autofluorescence to the local chemical structure. Since quenching of autofluorescence does


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not require a recovery of the long -electron conjugation length, local deoxygenation can lead to immediate quenching. It is possible that, by the time the reduction of lower energy oxides is completed, a significant portion of the autofluorescence has been already quenched. In this case, the measured [AF] appears to stay at a low value for the latter part of the reaction. Reduction of higher energy oxides and stabilization of neighboring oxides by the reduction of an oxide group have only small effects on the remaining [AF]. The same argument also applies to the pseudofirst order model even if it is the case. Additionally, it should be noted that [AF] is a sum of all fluorescence originating from many elementary reactions and processes. According to our scheme, [AF] diminished significantly while the low activation energy oxides were reduced. Their energetic variations are small enough to be averaged out and the resulting mean value is observed to follow simple kinetics. Although it seems accidental, the fact12,13 that many different kinds of contaminant adsorption on heterogeneous surfaces, when the amounts are measured as average values, follow the pseudosecond order model suggests a possibility that it is a necessary consequence of complex systems. In addition to oxidation of graphite flakes, the Hummers’ method also produces small oxidative debris that may stay adhered on GO surfaces.31, 32 The carbonaceous debris is heavily oxidized and its size is estimated to be on the order of a few nm. The debris is one of the sources of the autofluorescence band around 470 nm28 and can induce excimer emissions at longer wavelengths by interacting with the graphene plane30. In the present case, TwiF spectroscopy has revealed that only a few GO sheets have the 470 nm band while others do not. Nevertheless, as indicated by the previous studies, we verified that washing GO with NaOH diminished the ensembleaveraged autofluorescence. These results suggest that some of our GO sheets have been covered by the debris and the measured [AF] contains contributions from both the debris as well as GO


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sheets. Although the nanometer-sized debris floating freely in the reacting solution should react much faster than the measurement time scale in this experiment, those adhered on over 100 m2 GO sheets may take a much longer time to react due to a decreased collision frequency with LAA molecules. We cannot reject a possibility that the reduction of oxides present on the debris has affected the kinetics. As for a possible effect of the debris on the dye experiment, both autofluorescence intensity under 600 nm excitation and visible absorbance are several orders of magnitude smaller than those of the dye. Thus, the effects of the debris are negligible in the dye experiment. The pseudo-second order model (under the excess L-AA concentration) implies that the rate of [AF] decrease is given by

―

𝑑[𝐴𝐹] 𝑑𝑡

= 𝑘2[𝐴𝐹]2

(8)

None of the autofluorescence mechanisms proposed so far18-30 involves an action of the same species twice in a process. Then, if we assume that [AF] is proportional to the average concentration of oxides and perturbed local -electron densities responsible for autofluorescence, there must be two different kinds of oxides and related perturbations. In this case, (8) can be rewritten as

―

𝑑[𝐴𝐹] 𝑑𝑡

= 𝑘2′[𝑂𝑥1][𝑂𝑥2]

(9)

where [𝑂𝑥1] and [𝑂𝑥2] are the average concentration of each kind, both contributing to [AF]. The result of ATR-FTIR given in Figure 1b suggests that these oxides are the hydroxyl and epoxy groups.


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(b)

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-1

k (s )

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1.0

1.0

0

500 Area (m2)

1000

1

2 Layer

3

>4

Figure 7. Dependence of the pseudo-second order rate constant on (a) area and (b) a number of layers.

The area and layer number dependences of the rate constant are shown in Figure 7. The rate appears to be larger for the smaller area sheets, in good agreement with the results of TwiF microscopy with R6G. It is independent of the layer number, confirming our expectation that only the outermost surface exposed to the excitation beam fluoresces due to the strong UV absorption of GO. The same dependences were also found on the 3-parameter pseudo-first order rate constant (not shown). Area Dependence of Rate Constant. Both TwiF studies with dye and autofluorescence suggest that the rate constant is larger for smaller area GO sheets. This means that a small sheet deoxygenates and recovers -electron conjugation faster. Since all sheets in a given set are located within a single TwiF imaging area, the local reaction condition should be the same in each set. All sheets have flat polygonal shapes, thus morphology does not play a role. The reaction taking over tens of minutes in the low viscous mixture indicates that molecular diffusion


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is not a limiting process. Since each molecule of L-AA is far smaller than each GO sheet, the area difference on this scale should be insignificant for the L-AA molecule. Then, we must conclude that the smaller area sheets had fewer numbers of oxides and less damaged -electron conjugation before the reduction reaction had started. Macroscopic GO sheets with different reactivity are likely caused by the Hummers’ reaction and the sonication process. The sonication process that is used to exfoliate the oxidized graphite flakes to individual GO sheets in a liquid is very violent. It not only exfoliates but also tears a GO sheet apart into small pieces.5 In order to minimize its effect and obtain large area sheets, we kept the sonication process to 5 s. The average diameter of the starting graphite flakes was 60 m. This means that the smaller area sheets are the first several pieces that have been separated from the oxidized graphite flakes. A single sheet in the original graphite flake consists of many crystallographic domains of submicron sizes (Figure 8). Some domains are connected smoothly while others have many defects along boundaries. Here, a smooth connection refers to energetically stable linkages such as non-hexagonal carbon-carbon bonds. If we assume that the oxidation reaction in the Hummers’ method starts from defects that have initially existed in the

Figure 8. Schematic illustrating a domain structure of an oxidized graphite sheet with smoothly connected domain boundaries (thin lines) and heavily defective boundaries (thick lines). When a line connected by the defective boundaries encloses a piece, it can be exfoliated. ACS Paragon Plus Environment

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original graphite flake, the smoothly connected domains have a smaller probability of being oxidized and the defective domain boundaries should be heavily oxidized. Since mechanical weakness is often associated with heavy oxidation, a line traced by the heavily oxidized boundaries experiences the strongest stress during sonication and initiates tearing. The oxidized line, however, needs not to enclose the whole piece completely and may terminate at the middle. A piece cannot be freed unless the oxidized line forms a closed perimeter. We expect that a number of the terminated oxidized lines increases as the area becomes larger because the oxidized line needs to trace a longer distance to enclose a large piece completely. Thus, a smaller area piece contains less oxidized parts at the earlier period of sonication. As a support of this scheme, large GO pieces with several “wedges”, which are interpreted as the remainder of the terminated oxidized lines, have been seen in TwiF observation.5 Also, it is consistent with the result of the Elovich model that the area of low activation energies is localized. As the sonication continues, less oxidized lines are mechanically damaged further and eventually induce tearing. The separated pieces become smaller and differences between the degrees of original oxidation will be overwhelmed by the edge effects. We think that the sonication time of around 5 s is long enough to exfoliate but short enough to keep the original property of GO sheets. Unfortunately, nearly all previous studies have not described experimental details. We suspect that most studies have applied sonication for longer than a minute. In this case, GO sheets have been torn into submicron-sized pieces, and random mixing of such pieces has hindered heterogeneity of the original oxidized graphite flakes.

Conclusions


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The TwiF study with R6G indicates the Elovich model whereas the autofluorescence study results in the pseudo-second order model. The former experiment probes a recovery of the electron conjugation length over multiple hexagonal units, while the latter experiment follows local deoxygenation. As the reaction continues, recovering the long conjugation length becomes increasingly difficult but local deoxygenation proceeds with a constant barrier. Even though a single reducing agent is used, there are so many kinds of elementary reactions taking place simultaneously on a heterogeneous surface that kinetics appear different depending on which processes we follow. As far as restoring the original -electron conjugation of graphene is concerned, the Elovich model is consistent with the present situation that no reduction reaction has never succeeded. We think that violent oxidation by the Hummers’ method is responsible for the reduction reaction with increasing activation energies. For better restoration, mild oxidation methods are highly desirable.

AUTHOR INFORMATION Corresponding Author * [email protected] ORCID Masahito Sano: 0000-0002-9050-5269 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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A part of this work is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

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TOC Graphic


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Reaction Kinetics of Reducing Graphene Oxide at Individual Sheet

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