Regulating the Catalytic Function of Reduced Graphene Oxides Using

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Regulating the Catalytic Function of Reduced Graphene Oxides Using Capping Agents for Metal-Free Catalysis Jaeeun Song, Shin Wook Kang, Young Wook Lee, Yangsun Park, Jun-Hyun Kim, and Sang Woo Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13970 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 25, 2016

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ACS Applied Materials & Interfaces

Regulating the Catalytic Function of Reduced Graphene Oxides Using Capping Agents for Metal-Free Catalysis Jaeeun Song,†,§ Shin Wook Kang,†,§,¶ Young Wook Lee,† Yangsun Park,† Jun-Hyun Kim,†,‡ and Sang Woo Han*,† †

Center for Nanotectonics, Department of Chemistry and KI for the NanoCentury, KAIST, Daejeon 34141, Korea ‡

Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, USA

ABSTRACT: Reduced graphene oxide (rGO) functionalized with organic capping agents has gained increasing attention as a promising metal-free catalyst. To optimize the properties of rGO for target applications, comprehending the link between the catalytic function of rGO and the chemical and structural characteristics of capping agents is critical. Herein, we report a systematic study on the effect of capping agents on the catalytic function of rGO for redox reactions using nitrogen-containing surface modifiers with distinctly different chemical structures, such as poly(diallyldimethylammonium chloride), cetyltrimethylammonium chloride, and poly(allylamine hydrochloride), which have the capability to endow rGO with improved suspension stability, enhanced reactant adsorption, and modified electronic properties. Functionalized rGOs were facilely prepared by the reduction of graphene oxide with hydrazine in the presence of the capping agents. The results of model redox reactions, i.e., 4-nitrophenol and ferricyanide reduction reactions, catalyzed by the functionalized rGOs corroborated that the way the capping agents functionalize rGO, which is highly correlated with their chemical structure, drastically influences the overall reaction kinetics, including induction time, reduction rate, total reaction time, and reaction order. This strongly suggests that the judicious selection of capping agents is crucial to fully harness the catalytic function of rGO and thus to design novel rGO-based non-metallic catalysts with controllable reaction kinetics.

KEYWORDS: reduced graphene oxide, capping agent, reduction reaction, catalysis, metalfree catalyst

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INTRODUCTION Recently, significant attention has been paid to the development of metal-free catalysts to address cost and environmental issues that are commonly associated with metal-based catalyst systems.1-5 Among alternatives to precious metal catalysts, reduced graphene oxide (rGO) is of particular interest due to its extraordinarily large surface area and rich catalytically-active sites, including conductive sp2-conjugated carbon domains, zigzag edges, and structural defects, such as oxygen functionalities, topological defects, and defect holes.6,7 It has been actively tested as a non-metallic catalyst in various chemical and electrochemical reactions.7-12 Furthermore, the modification of the rGO surface by heteroatom doping with electron-withdrawing elements and/or surfactants can allow the manipulation of its electronic and catalytic properties for designing more promising catalytic materials.13,14 One of the critical issues in the processing and practical application of rGO is its tendency to form irreversible aggregates due to intersheet hydrophobic interactions especially during its preparation via the chemical reduction of graphene oxide (GO).15 In this regard, various capping agents including organic molecules and polymers have been employed to obtain a good dispersion of rGO, and thus to fully harness its large surface area.16 As the proper surface modification of rGO can further give control over its chemical and electronic properties, the incorporation of capping agents into rGO should offer an opportunity to optimize their properties for target applications.16-26 Indeed, functionalized rGO materials have been widely utilized in the fabrication of functional films for electronic devices,18-20 sensors,21-23 and electrodes,24-26 and they have attracted tremendous interest as a promising metal-free catalyst.7,16,17 To gain fine control over the function of functionalized rGO in catalysis and thus to develop an efficient catalyst system, clarifying the correlation of the catalytic properties of rGO with the chemical and structural characteristics of capping agents is critical. 2 ACS Paragon Plus Environment

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Here, we report a systematic study on the effect of capping agents on the catalytic function of rGO using various nitrogen-containing polymer and surfactant molecules with distinctly different characteristics. The functionalization of rGO was achieved by the reduction of GO with hydrazine in the presence of poly(diallyldimethylammonium chloride) (PDDA), cetyltrimethylammonium chloride (CTAC), or poly(allylamine hydrochloride) (PAA). These representative positively-charged nitrogen-containing capping agents with different chemical structures were chosen for the surface-modification of rGO due to our expectation that they would impart to rGO improved suspension stability, enhanced reactant adsorption, and modified electronic properties through charge transfer. To elucidate the link between the catalytic function of rGO and the chemical structure of capping agents, we thoroughly investigated the catalytic properties of the functionalized rGOs toward model redox reactions, such as 4-nitrophenol (4-NP) and ferricyanide reductions. The reaction kinetics, i.e., induction time, reduction rate, total reaction time, and reaction order, substantially depended on the type of capping agents, which could be correlated with the inherent chemical structure of the capping agents. This emphasizes the importance of careful choice of capping agents to fully harness the catalytic function of rGO. The present study can thus give a rational guideline for the designing and selection of capping agents pertinent to intended catalytic reactions and will contribute to devise efficient rGO-based metal-free catalysts.

EXPERIMENTAL SECTION Chemicals and materials. Graphite (Samjung CNG), sodium nitrate (NaNO3, 99.0%, Aldrich), potassium permanganate (KMnO4, 99.0%, Aldrich), hydrogen peroxide (H2O2, 30 wt%, Daejung Chemicals & Metals Co.), PDDA (MW = 100,000-200,000, 20 wt% in water, Aldrich), CTAC (25 wt% in water, Aldrich), PAA (MW = 58,000, Aldrich), hydrazine



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hydrate (N2H4·H2O, 64%, Aldrich), sodium borohydride (NaBH4, 99%, Aldrich), potassium ferricyanide (K3Fe(CN)6, 99+%, Sigma-Aldrich), sodium thiosulfate (Na2S2O3, 99.99+%, Aldrich), and 4-NP (HPLC grade, Fluka) were all used as received. Ultrapure Millipore water with a resistivity of greater than 18.0 MΩ·cm was used in the preparation of aqueous solutions. Preparation of GO. GO was prepared from graphite based on the modified Hummer’s method.27,28 In a typical synthesis of GO, 1 g of graphite and 0.5 g of NaNO3 were mixed in a beaker containing 30 mL of concentrated H2SO4, and this beaker was then placed in an ice bath. 5 g of KMnO4 was slowly added to this solution, while the temperature was kept under 10 °C. The mixture was then gradually heated to 35 °C in a water bath and stored at that temperature for 2 h (Caution: The temperature of the mixture must be kept under 50 °C to prevent its ignition.). Then, the reaction mixture was placed in the ice bath and 40 mL of water was slowly added to this mixture. The reaction mixture was stirred for additional 1 h in the water bath. Finally, 100 mL of water was poured into the mixture and 30% H2O2 was then added until bubbles were no longer generated. The resultant mixture was filtered and washed with water until the pH of filtrate solution reached to 7. The prepared GO was dried at room temperature and used in the following experiments. Preparation of bare and functionalized rGO. Bare rGO was prepared by following the reported procedure.29 First, GO powder was dispersed in 20 g water (0.05 wt%), and then subjected to ultrasonication for 1 h to evenly disperse GO in water. Then, 5 mL of the mixture was diluted with 5 mL of water in a vial. 5 µL of 35% hydrazine and 35 µL of 28 wt% ammonia solution were then added to the reaction mixture. The resultant mixture was shaken vigorously for 5 min, and then heated to 95 °C in a water bath and maintained at that temperature for 1 h. For the preparation of functionalized rGO, the reduction of GO was proceeded in the presence of capping agents. The amount of the capping agents was 5-fold 4 ACS Paragon Plus Environment

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larger than that of GO in the dispersion. Before the reduction of GO, the reaction mixture was stirred overnight to allow GO and capping agents to fully interact. On the contrary to the case of bare rGO, where ammonia solution was employed to make a stable aqueous dispersion of rGO, the addition of ammonia solution was not required in the preparation of functionalized rGO, because the capping agents can make the rGO surfaces positively charged, and thus endow rGO with good suspension stability (vide infra). The prepared bare and functionalized rGO dispersions were stored under ambient conditions for at least 72 h to remove the volatile hydrazine. Catalysis experiments. For the catalytic reduction of 4-NP, 200 µL of the prepared bare or functionalized rGO solution and 20 µL of 10-2 M 4-NP were placed in a quartz cuvette with a 1 cm path length containing 2.5 mL water. The catalytic reaction was initiated upon the addition of 300 µL of 10-1 M NaBH4 aqueous solution. The progress of the reaction was monitored by measuring the absorbance of 400 nm peak of 4-nitrophenolate, which is formed by the deprotonation of 4-NP in NaBH4 medium, with UV-vis spectroscopy. The catalytic reduction of ferricyanide was carried out with bare and functionalized rGO using sodium thiosulfate as a reducing agent. Specifically, 50 µL of bare rGO or PDDA-modified rGO catalyst was added to 0.95 mL water containing 0.5 mL of 2.5 mM K3Fe(CN)6. The catalytic reaction was initiated by adding 1 mL of 10-1 M Na2S2O3 aqueous solution. Then the change in the absorbance of 420 nm peak associated with ferricyanide was monitored by UV-vis spectroscopy. Characterization. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV. Atomic force microscopy (AFM) images and height profiles were obtained in air at room temperature by using a Park System XE-100 scanning probe microscope. X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using Cu Kα (0.1542 nm) 5 ACS Paragon Plus Environment


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X-ray radiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo VG Scientific Sigma Probe spectrometer equipped with Al Kα X-ray (1486.6 eV). XPS spectra were calibrated using the C 1s peak at 284.4 eV. UV-vis spectra were recorded using a Shimadzu UV-3600 UV-vis-NIR absorption spectrometer. Zeta potential measurements were carried out with a Malvern Zetasizer (Nano ZS). Raman spectra were obtained on a Jobin Yvon/HORIBA LabRAM spectrometer equipped with an integral microscope (Olympus BX 41). The 632.8 nm line of an air-cooled He/Ne laser was used as an excitation source. Raman scattering was detected with 1800 geometry by using a thermoelectrically cooled 1024 × 256-pixel charge coupled device (CCD) detector. The laser beam (1.5 mW) was focused onto samples with an objective lens (×50, NA = 0.50). Data acquisition times were usually 10 s. The holographic grating (1800 grooves mm-1) and the slit allowed a spectral resolution of 1 cm-1. The Raman band of an Si wafer at 520 cm-1 was used to calibrate the spectrometer. Fourier transform infrared spectroscopy (FTIR) analyses were carried on a Cary 600 series spectrophotometer with a spectral resolution of 4 cm-1. Each sample was thoroughly mixed with KBr to prepare a pellet prior to analysis.

RESULTS AND DISCUSSION AFM and TEM images of GO sample prepared by the modified Hummer’s method shown in Figures 1a and b, respectively, exhibit the typical features of GO sheets with fully exfoliated layers. The thickness of the GO sheets was about 1.2 nm, which was estimated from the AFM height profile measurements (Figure 1c), further indicating the formation of single-layer hydrated GO sheets.30 The aqueous dispersion of the GO had a homogeneous brownish color, which also demonstrated the highly oxidized states of the GO sheets (Figure 1d). The zeta potential of the GO sample was -0.35 ± 0.2 mV. The prepared GO was subjected to hydrazine reduction to obtain rGO. As mentioned above, rGO was prone to aggregate during its 6 ACS Paragon Plus Environment

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preparation in the absence of capping agents due to the hydrophobic nature of the restored sp2-conjugated carbons. Indeed, as shown in Figure 1d, rGO prepared without any additives showed poor dispersion in water (pH = 7), forming irreversible aggregates during the reduction process. In contrast, rGO samples prepared by the reduction of GO with hydrazine in the presence of PDDA, CTAC, and PAA showed stable dispersion in water, thus facilitating their application in catalysis and preserving the unique property of the individual rGO sheets (Figure 1d). Hereafter, we will refer to the rGO prepared with PDDA, CTAC, and PAA as PDDA-rGO, CTAC-rGO, and PAA-rGO, respectively. Zeta potential measurements of the suspensions of the functionalized rGOs corroborated the successful surface modification of rGO with the capping agents, which made the rGO surfaces positively charged (Table 1). The good suspension stability of the functionalized rGOs can thus be attributed to electrostatic repulsion between positively-charged rGO sheets. To compare the catalytic function of the functionalized rGOs with that of bare rGO, and thus to elucidate the effect of capping agents on the rGO-catalyzed reactions, an aqueous dispersion of bare rGO was also prepared by employing ammonia solution during the reduction of GO to make high pH conditions (pH ≈ 10) [Figure S1 in the Supporting Information (SI)]. This can induce strong electrostatic repulsive interactions between individual layers of rGO due to the full ionization of residual carboxyl groups at the edges of rGO sheets.29 Apparently, zeta potential measurement of the suspension of bare rGO prepared at high pH showed that the rGO surface was negatively charged (Table 1). Raman spectroscopy and XRD analysis were conducted to verify the reduction of GO. The increase in the relative intensity ratio of the D peak to the G peak (ID/IG) for both the bare and functionalized rGO samples compared to the GO in the Raman spectra indicated the formation of more graphitic domains, supporting the successful reduction of the GO (Figure 2a).15,31 The ID/IG values of the GO, rGO, PDDA-rGO, CTAC-rGO, and PAA-rGO were 0.93, 7 ACS Paragon Plus Environment


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1.24, 1.04, 1.08, and 1.07, respectively. The XRD patterns of graphite, GO, rGO, and functionalized rGOs are shown in Figure 2b. Graphite showed a (002) diffraction peak at 26.46°, which corresponds to an interlayer d-spacing of 3.37 Å, whereas the GO showed a (001) diffraction peak at 10.44°, which corresponds to an interlayer d-spacing of 8.47 Å. The increased d-spacing of the GO relative to that of the graphite is due to the formation of oxygen-containing functional groups and the accommodation of water molecules between GO sheets.32,33 On the contrary, the XRD pattern of the bare rGO showed a broad diffraction peak at 24.84° (d-spacing = 3.59 Å), which is positioned close to that of the graphite. This is due to the removal of the oxygen-containing functional groups during the reduction process. All the XRD patterns of the functionalized rGOs had a similar feature to that of the bare rGO, implying that GO sheets were successfully reduced regardless of the presence of capping agents.34 The calculated interlayer d-spacings of the PDDA-rGO, CTAC-rGO, and PAA-rGO were 4.02, 4.10, and 3.93 Å, respectively. The slightly increased d-spacings of the functionalized rGOs relative to the bare rGO further indicated the proper surfacemodification of rGO with the capping agents. Furthermore, the similar interlayer d-spacing values of the functionalized rGOs demonstrated that the amount of capping agent associated to rGO is similar in the three cases. In addition, the presence of capping agents associated to rGO was also confirmed by the FTIR spectra of the purified functionalized rGOs (Figure S2 in the SI). XPS measurements of the GO, rGO, and functionalized rGOs were further carried out to investigate changes in their chemical structures during reduction. Comparison of XPS survey spectra of the GO and rGO samples showed a great increase in the C/O atomic ratio of the rGO relative to that of the GO, demonstrating the substantial removal of oxygen-containing functional groups during the reduction of the GO (Figure S3a in the SI). In fact, the C 1s XPS spectrum of the rGO exhibited the strong suppression of epoxy groups on the basal planes of 8 ACS Paragon Plus Environment

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the GO, further confirming the reduction of the GO (Figures S3b and c in the SI). Notably, N 1s XPS peaks around 400 eV were also detected both for the GO and rGO, indicating the introduction of nitrogen to the GO and rGO sheets. The N-doping levels of the prepared GO and rGO were estimated to be 1.81 and 3.06 atom%, respectively. The N-doping might be due to the use of NaNO3 and hydrazine in the oxidation process for the GO and the reduction process for the rGO, respectively.29,35 Figure 3a shows the XPS survey spectra of the functionalized rGOs. Similar to the rGO, the C/O atomic ratio of the functionalized rGOs substantially increased compared to that of the GO. Furthermore, a significant decrease in the carbon composition associated with epoxy groups was observed in each C 1s XPS spectrum of the functionalized rGO in comparison to that of the GO (Figure 3b-d). These results indicate that the reduction of the GO proceeded effectively in the presence of the capping agents as revealed by Raman and XRD measurements. The N 1s peaks around 400 eV were also observed in the XPS survey spectra of the functionalized rGOs. However, the exact Ndoping levels of the rGO sheets could not be evaluated due to the presence of the nitrogencontaining capping agents. To decipher the effect of capping agents on the catalytic function of rGO, we chose a redox reaction, i.e., the reduction of 4-NP to 4-aminophenol (4-AP) by NaBH4, as a model catalytic reaction in this work. In general, the reduction of 4-NP is catalyzed by metal-based nanocatalysts36-46 and metal oxides.47,48 For the mechanism of the catalytic reduction of 4-NP by NaBH4, it has been widely accepted that both BH4- and 4-NP adsorb on the surface of catalysts to proceed the reaction (Langmuir-Hinshelwood model).36,49 In this mechanism, BH4- ions react with the catalyst surface and transfer a surface hydrogen species, hydride. Concurrently, 4-NP molecules adsorb on the surface of catalysts. The adsorbed 4-NP could then be reduced by the surface hydrogen species to yield the final product, 4-AP, during which the adsorbed hydrogen species releases proton and electron that are transferred to 4-NP 9 ACS Paragon Plus Environment


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to proceed the reduction. In the catalysis, the catalyst plays multiple roles; it provides the adsorption sites for reactant molecules and functions as an electron reservoir for hydride released from BH4-, as well as relays electrons to 4-NP. In this regard, it can be expected that rGO could be a promising alternative to precious metal-based nanocatalysts as it can facilitate the adsorption of chemical species involved in the reaction due to its rich in edges, diverse structural defects, and π-system planes, and can efficiently reserve electrons and mediate electron transfer through conductive sp2-conjugated carbon domains.11,50-53 In conjunction with these inherent characteristics of rGO, which can be synergistically coupled in catalytic reactions, the modification of rGO with positively-charged nitrogen-containing capping agents could endow rGO with extra catalytic functions, such as increased stability, promoted charge transfer, and enhanced reactant adsorption, the extent of which might depend on the chemical structure of the capping agents. For comparative purpose, the catalytic properties of bare rGO toward the 4-NP reduction were investigated first. Figure 4a shows successive UV-vis extinction spectra (time interval = 3 min) of the 4-NP reduction reaction in the presence of the rGO after the introduction of excess NaBH4. After 10 min of induction time, the characteristic absorption peak of 4nitrophenolate ions, which is the deprotonated form of 4-NP in NaBH4 medium, at 400 nm gradually decreased in intensity as the reaction progressed, and it disappeared after 54 min of reaction, indicating that rGO can effectively catalyze the reduction reaction. The isosbestic points were clearly observed at 225, 245, 281, and 314 nm, further demonstrating that 4-AP was the only product of the reaction.43 The long induction period was required for the initial adsorption of reactant molecules at the rGO surfaces and the surface restructuring of the rGO to initiate the reaction due to the negatively-charged surfaces of the rGO catalyst.36,54 Notably, as shown in Figure 4b, a linear relationship was found between the absorbance at 400 nm and reaction time after the induction period (slope = 2.90 × 10-2 min-1), where the 10 ACS Paragon Plus Environment

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absorbance values were corrected by the subtraction of the absorbance of the pure rGO dispersion at 400 nm from that of the reaction solution (Figure S4a in the SI). On the contrary, the logarithm of the absorbance at 400 nm relative to that at the initial stage, ln(A/A0), showed a poor linear correlation with the reaction time (Figure S4b in the SI). These findings indicate that the 4-NP reduction catalyzed by the rGO followed pseudo-zeroorder kinetics with a rate constant, k, of 1.67 × 10-6 M min-1, which is in sharp contrast with the findings of previous studies on the reduction of 4-NP with metal-based nanocatalysts that follows pseudo-first-order kinetics. This can be due to the limited number of active sites in nitrogen-doped rGO, as Kong et al. reported.8 Interestingly, although the N-doping level of the prepared rGO (C/N = 27) was much lower than that of the reported N-doped graphene (C/N = 8), the activity factor, i.e., rate constant normalized to the total mass of catalysts,46,50 of the present rGO catalyst for the 4-NP reduction (3.35 × 10-5 M min-1 mg-1) was comparable to that of the N-doped graphene (3.22 × 10-5 M min-1 mg-1).8 This could be attributed to the better dispersibility of the prepared rGO catalyst in solution than the N-doped graphene. Figure 5 shows the time-dependent UV-vis extinction spectra and kinetic profiles of the 4NP reduction reactions with the functionalized rGOs. The order of kinetics, rate constant, induction time, and total reaction time for each functionalized rGO are summarized in Table 1 together with those of the bare rGO. As shown in Figure 5 and Table 1, the catalytic function of rGO highly depended on the type of capping agent. Noticeably, the PDDA-rGO catalyst showed the best catalytic activity among the various functionalized rGOs. In sharp contrast to the case of the bare rGO, the reaction with the PDDA-rGO catalyst started immediately after the addition of NaBH4 (Figure 5a), indicating that the positively-charged rGO surface can facilitate the adsorption of 4-nitrophenolate and borohydride ions to the rGO surfaces, thus allowing the prompt initiation of the reaction. Apparently, the reaction followed pseudo-zeroorder kinetics and terminated after 36 min (Figures 5a, d, and g). From the slope of the 11 ACS Paragon Plus Environment


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absorbance at 400 nm vs. reaction time, 3.44 × 10-2 min-1 (Figure 5d), the k and activity factor of the reaction catalyzed by the PDDA-rGO were estimated to be 1.98 × 10-6 M min-1 and 3.96 × 10-5 M min-1 mg-1, respectively, demonstrating its faster reaction kinetics compared to that with the bare rGO. The reduction of 4-NP did not proceed in the absence of either NaBH4 or the PDDA-rGO, and PDDA itself showed no catalytic activity (Figure S5 in the SI), verifying that PDDA can indeed accelerate reaction kinetics through combination with rGO. It can be posited that the functionalization of rGO with positively-charged PDDA having election-withdrawing capability could increase the positive-charge density on the rGO plane through the charge-transfer between PDDA and adjacent carbon atoms in the rGO,16,17 thus enhancing the reactant adsorption and catalytic process. In fact, the electron-withdrawing effect of PDDA was confirmed by the observation of the blue-shift in the Raman G band of the PDDA-rGO by 5 cm-1 compared to that of the bare rGO (see the dotted line in Figure 2a).55-57 Compared to the case of the PDDA-rGO, a much longer reaction time including 3 min induction time was required to terminate the reaction with the rGOs functionalized with other nitrogen-containing capping agents, CTAC-rGO and PAA-rGO (Figure 5b and c and Table 1), although their surfaces are positively charged and nitrogen atoms in the capping agents may endow rGO planes with increased positive-charge density as in the case of the PDDA-rGO. Furthermore, unlike the reactions catalyzed by the rGO and PDDA-rGO, fairly good linear relationships were found between ln(A/A0) and reaction time after the induction period (Figures 5h and i), indicating that the reactions catalyzed by the CTAC-rGO and PAA-rGO followed pseudo-first-order kinetics. The k values of the reactions with the CTAC-rGO and PAA-rGO were 2.94 × 10-2 and 2.84 × 10-2 min-1, respectively. These results explicitly demonstrate that the chemical structure of the capping agent has a profound influence on the catalytic function of rGO. The inferior catalytic activity and change in the order of kinetics of 12 ACS Paragon Plus Environment

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the CTAC-rGO compared to the PDDA-rGO might be attributed to the distinct structural characteristics of its CTAC passivation layer. A quaternary ammonium surfactant with a long alkyl chain, such as CTAC, can readily form a bilayer structure around a solid support, and make it positively charged.58,59 As such, the CTAC bilayer formed on the rGO plane can impede the diffusion of the reactant molecules into the catalytic sites on the rGO surface, thus retarding the reduction reaction. As a result, a longer induction period and a longer reaction time should be required to initiate and complete the 4-NP reduction with the CTAC-rGO compared to the PDDA-rGO. In addition, a higher ratio of the number of active sites at the rGO plane to the number of incoming reactant molecules in comparison to those of the rGO and PDDA-rGO due to the limited diffusion of the reactants may cause the order of kinetics to change from pseudo-zero-order to pseudo-first-order.37 On the other hand, the sluggish reaction rate and the switching of the order of kinetics for the case of the PAA-rGO catalyst are thought to be associated with the tendency of PAA to make covalent bonds with GO sheets through the ring opening reaction between its amine groups and epoxy groups in GO.60-62 The covalently-bonded PAA layer on the rGO sheets could thus act as a diffusion barrier for the incoming reactants, as in the case of the CTAC-rGO. These results and findings collectively verify that the catalytic function of the functionalized rGOs is strongly dependent on the way the capping agents passivate rGO surfaces, and the employment of nitrogen-containing capping agents without a disposition to form a bilayer or a specific chemical bond on rGO planes as surface-modifiers is promising for the improvement of the efficiency of rGO-based catalysts. To gain a further insight into the effect of the chemical structure of capping agents on the catalytic function of rGO, the catalytic properties of rGO functionalized

with

other

nitrogen-containing

capping

agent,

such

as

branched

poly(ethylenimine) containing primary, secondary, and tertiary amine groups, were tested, but it showed poor catalytic activity due to the formation of irreversible aggregates. In addition, 13 ACS Paragon Plus Environment


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rGO functionalized with a negatively-charged capping agent, such as poly(styrene sulfonate), exhibited significantly lower catalytic activity with a much longer induction period compared to that of the rGO functionalized with the positively-charged capping agents (Figure S6 in the SI). These results further corroborate the importance of the judicious selection of capping agents. The 4-NP reduction reactions at different temperatures were also carried out with the PDDA-rGO catalyst to compare its activation energy (Ea) with those of metal-based nanocatalysts. From the slope of the Arrhenius plot (Figure S7 in the SI), the Ea of the PDDA-rGO catalyst was estimated to be 37 kJ mol-1. Notably, this value is comparable to those of metal-nanostructured catalysts (Table S1 in the SI). Although the direct comparison of our Ea value with those found in other systems is difficult due to the different interplay between adsorption and kinetic constants,36,41 this result demonstrates that the functionalized rGO can be a promising alternative to metallic nanocatalysts. Furthermore, the proposed functionalized rGO can also be applied as an efficient catalyst in other redox reaction, such as ferricyanide reduction by thiosulfate, 2Fe(CN)63- + 2S2O32- → 2Fe(CN)64- + S4O62-, which is commonly catalyzed by metal-based nanocatalysts.63,64 As shown in Figure 6a, the absorbance peak of ferricyanide ions at 420 nm gradually decreased in intensity in the presence of the PDDA-rGO without an induction period, indicating the successful reduction of ferricyanide to ferrocyanide. Contrary to the cases of metal-based nanocatalysts, the reaction followed pseudo-zero-order kinetics with a k value of 2.99 × 10-6 M min-1 (Figures 6b and c), which can be ascribed to the limited number of active sites in the PDDA-rGO, as we previously discussed. The same reaction catalyzed by the bare rGO exhibited much slower reaction kinetics (k = 1.65 × 10-6 M min-1) compared to the PDDA-rGO (Figures 6df). Indeed, the reduction of ferricyanide did not effectively proceed in the absence of either rGO or PDDA-rGO (Figure S8 in the SI). These results, taken together, suggest that the 14 ACS Paragon Plus Environment

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present study can provide a new direction for the development of efficient carbon-based nanocatalysts as promising alternatives to precious metal-based catalysts.65-68

CONCLUSIONS In summary, we presented a facile synthetic approach for the functionalization of rGOs with various nitrogen-containing capping agents, i.e., PDDA, CTAC, and PAA, where GO was reduced by hydrazine in the presence of the capping agents to yield functionalized rGOs. The functionalized rGOs could readily be applied in redox reactions as non-metallic catalysts. Through systematic studies, we found that the distinctive way the capping agent functionalizes the rGO surface determines the catalytic properties of rGO, verifying the importance of selecting a proper capping agent to have fine control over the catalytic function of rGO. We envision that the proposed approach can provide a promising opportunity for the functionalization of pre-modified rGO, like heteroatom-doped rGO, without destroying its intrinsic structural characteristics, thereby extending the capability of rGO to various applications, and that it can be extended to the designing of a new class of metal-free catalysts.



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ASSOCIATED CONTENT Supporting Information Additional experimental data (Figures S1-8) and activation energy of various catalysts for 4NP reduction (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

[email protected]

Author Contributions §

These authors contributed equally to this work.

Present Addresses ¶

Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-Ro, Yuseong-Gu,

Daejeon 34129, Korea.

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1A3A2033469) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP).


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Table 1. Zeta Potential, Order of Kinetics, Rate Constant, Induction Time, and Reaction Time for Various rGO-Based Catalysts catalyst rGO

zeta potential (mV) -52.7 ± 3.2

order of kinetics pseudo-zero

rate constant, k 1.67 × 10-6 M min-1

PDDA-rGO

51.1 ± 5.8

pseudo-zero

1.98 × 10-6 M min-1

0

36

pseudo-first

-2

-1

3

66

-2

-1

3

72

CTAC-rGO PAA-rGO

48.3 ± 13.4 22.6 ± 4.7

pseudo-first

2.94 × 10 min 2.84 × 10 min


induction time (min) 10

reaction time (min) 54

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Figure 1. (a) Tapping mode AFM and (b) TEM images of GO sheets. (c) AFM height profiles along the lines in (a). (d) Photograph of the suspensions of GO, rGO, PDDA-rGO, CTACrGO, and PAA-rGO.



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Figure 2. (a) Raman spectra GO, rGO, PDDA-rGO, CTAC-rGO, and PAA-rGO. (b) XRD patterns of graphite, GO, rGO, PDDA-rGO, CTAC-rGO, and PAA-rGO.


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Figure 3. (a) XPS survey spectra of PDDA-rGO, CTAC-rGO, and PAA-rGO. Deconvoluted C 1s XPS spectra of (b) PDDA-rGO, (c) CTAC-rGO, and (d) PAA-rGO.



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Figure 4. (a) Successive UV-vis spectra of 4-NP reduction reaction (time interval = 3 min) in the presence of rGO catalyst and (b) corresponding time dependence of the absorbance at 400 nm.


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Figure 5. (a-c) Successive UV-vis spectra of 4-NP reduction reaction (time interval = 3 min) and corresponding (d-f) time dependence of the absorbance at 400 nm and (g-i) plot of ln(A/A0) vs. time in the presence of (a,d,g) PDDA-rGO, (b,e,h) CTAC-rGO, and (c,f,i) PAArGO. Yellow regions in (e,f,h,i) indicate the induction period.



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Figure 6. (a,d) Successive UV-vis spectra of ferricyanide reduction reaction (time interval = 3 min) and corresponding (b,e) time dependence of the absorbance at 420 nm and (c,f) plot of ln(A/A0) vs. time in the presence of (a-c) PDDA-rGO and (d-f) bare rGO.


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SYNOPSIS TOC


Regulating the Catalytic Function of Reduced Graphene Oxides Using

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